I don't know anyone who would use that phrase and if I did I would probably lose them pretty fast, right after I responded with, "Praise Allah!" It's the same as "Thank god!", right?

Funny story here:
I used to work in a call center. One random caller asked, "Have you been told today that Jesus loves you?" I responded, "Yes. He told me himself. Except he pronounces his name "Hey-Zeus" (as in the Spanish pronunciation). She hung up. It's fun to make racist, homophobic evangelicals uncomfortable. lol

I went with unprofessional.
There's no need to bring a religious tone into my buying something at their store. Again they assume I believe in some sort of god like they do.
Here's something funny I've seen, a cashier freak out when someone's total is $6.66. lol

When someone says have a blessed day I say "that's nice but I don't have a magical incantation for you. Will abracadabra work?

Frankly, there's no such expressions being used in French, so I can't really say it feels right or wrong, but if it annoys you, you can always say "and may the Flying Spaghetti Monster bless yours with its noodly appendage"

The reason it bothers me is not just because they are assuming I believe in their voodoo, but that they Do Not Use Their Brains. They are weak minded sheep, sharing a bit of their brainwashing with me. I don't have patience for illogical drivel.

Maybe it shouldn't, but it does. It sounds ultra Christian, which I am not, and I usually sense a feeling of superiority like look at me, I'm so pious that I can bless you. I don't need to be blessed.
Another that annoys me is when I pay at the register and they say "Have a good one." Sometimes I ask "One what?" or if I am in a good mood, "Why just one, can't I have a good two or three?" I know, stupid. It is right up there with "No problem."

It doesn't bother me too much, but I do think it's sort of unprofessional. I don't recall this being a thing more than about 10-15 years ago. What was wrong with "have a nice day"? Maybe I'll start replying with "Thank you. May you feel the warm noodly appendage on your shoulder..." just to fuck with them and confuse them a bit.

I take it for what it is. It's just another pleasantry some people are using to try to be nice. I say thank you and carry on.

I feel most people say it for lack of anything better to say. It's right up there with I will pray for you.

It is something a lot of us don't think about. When my wife was dying people would call and say they would pray for her (even though she asked for no tears or prayers). She would point to me and wiggle her fingers (she had a brain tumor that affected only her speech). That meant say something so I told one guy she did not want his prayers. He said it was just a figure of speech. My reply was "so you don't really mean what you say"! I have since become acutely aware of the hollow language people use and often gently mention how I feel.

I hate hearing it.

I am an anti theist and how dare someone push their horrific magical thinking on me. I do not start a fight over it, but I make it clear that a have a nice day will do, without the preaching. I recently took an UBER. The driver was very nice and the car was clean. I would have given him a five rating, but I gave him a one and I made a complaint. As I got out of the car, without having any discussion on the subject, he handed me religious literature. I didn't pay to go to church. It i snot only unprofessional, but it was insulting. I was a captive audience and could not decide to do business with someone else. I'm tired of atheists and agnostics saying that it doesn't matter. How about "let's hang black people" or "let's put Jews in camps"? They are both Christian organizations which are just sharing their philosophy, trying to share their message of hope, protecting you from Jews and blacks. Why would that upset anyone? I'm supposed to read a news story about Orange Nazi Trump speaking before a Christian organization which calls for my death, and then smile when someone ells me to have a blessed day? If I am dying from a horrible disease, should I have a blessed day? If my dog was hit by a car that morning, should I have a blessed day? It is very different from wishing me a good day. A blessed day means that you cruel god is somehow involved in my life. Wishing me a good day means that no matter what is going on, you hope that things go well or get better. May Satan lick your nipples and and tickle your anus. Now that sounds like a better day.

The only time I would have an issue with a religious phrase is if it's used in a snarky way.

Nope

I voted unprofessional.

I'll expand a bit on why it and "I'll pray for you" bother me. Firstly, I think there's an implied and unspoken "in Jesus's name" to both of these phrases. Secondly, I see this as an act of vanity, using their lord's name to glorify themselves instead of their god, which makes them hypocrites.

it Does bother me, but I hear it so often that I’ve become inured

To me, It implies one must believe there is someone out there to do the blessing.

If it is said as a sincere platitude it doesn't bother me how can you be angry when someone wishes you well?

While it might make me want to roll my eyes, typically it is a kindly intended platitude. I usually just graciously accept it.

It seems to me that the more it bothers you, the closer you are to religion, and will stay confused about where you stand, I hope I didn't offend with this thought.