I hear this expression said with the incorrect "could" more often than than with the correct "couldn't." I attribute this to one of the following:

- the speaker is using wry sarcasm, although the inflection is usually wrong.

- the speaker actually does care a bit.

- it's easier to say "could" or it's habit.

I try not to be a pedant about this, but often fall. Yeah, I'm fun at parties.