I think most fans are secretly more interested in seeing themselves in the characters than they are with any kind of narrative consistency.
Consider how many people declare their favorite Turtle as "the one who's just like me!" and it becomes pretty clear. They'd rather see them behave how they WANT them to act, moreso than how they actually would.
Lots of projection and such going on. Kinda silly.
Also, "But they watch TV!" is the standard catch-all defense when trying to explain why they'd act like human beings despite being Not That.