We dont limit the right of people because we think they may do harm to others, or we think they are to 'stupid' to be responsible.
In the following examples, 'stupid' is reference to persons with a lack of common sense, not in the mentally-handicapped sense.
Do we let stupid people vote? Yes. Would anyone argue against that? (Aside from trolls...) No. Everyone has that right.
Do we let stupid people have religions? A belief is more dangerous than any firearm could ever be, but those rights are uninhibited.
Who would enforce this? The government? (Aside from sterilization in the 50's) Our government has never 'declared' any too stupid to do anything.
Imagine if the government eroded the rights of anyone with 'undesireable' traits, in this case stupidity. Now, with politics the way they are, that would be expanded, and would create a class of 'inferior' persons, subject to the 'superior' government.
How is that any different than Nazi Germany?