I do not disagree.
That's not uncommon...
Why do people use double negatives in common speech?
I agree..
That's common..
Aren't these the same things?
Technically the examples you have given are not actual double negatives they are journalistic/political equivocations
"I do not disagree." does not mean "I agree" it is a weasel word way of say "I neither agree nor disagree,"
"That's not uncommon" does not mean "That is common" it is a way of hinting "something is happening regularly by not every day"
For example a full moon is not a common occurrence, but it not an every day occurrence either, it is not uncommon."
A true double negative is slightly different and is grammatically incorrect and usually contains at least one contraction.
"I didn't do nothing" for example means "I did something" but gives the impression of meaning "I did nothing"
"I won't use no double negatives" means "I will use double double negatives" while sounding like it should mean "I will not use any double negatives"
Perhaps the most famous one of all is the first line of the Pink Floyd song "The Wall"
"We don't need no education"
Dis and un both mean "not "correct? So technically, they are double negatives... The thing is, people "understand what Is being said.. I do understand your explanation though...
@Cutiebeauty I agree absolutely, they are technically double negatives, but employed in such a way as to be deceitful, what in journalist terms are called weasel words, because the appear to confine themselves in a tunnel of truth, but actually leave enough wriggle room to turn about and bite off your nose.
The political pundit and the politicians themselves love these sort of phrased comments because depending on the out come they can always agree with the victor and use the "No, I never said that" against detractors.
As to your second question, something can be common but that doesn't mean you agree with it.
I know I hate that! The worst one is in court when an attorney says "is it not true". My response would be "yes it is not true" LOL
Let me put it this way, do you think it is the same if you shake someone's hand while asking "agreed"? And the person that is shaking your hand replies "well...I don't disagree? instead of saying a categorical "agreed!"
Great example!