So, if popular culture approves of and encourages it, can't blame the cops too much for doing it despite it being merely illegal...
Yes, we can.
Indeed, we can, but unfortunately "we" don't. And that's the problem.
The "we" here is not the Slashdot readers (or other people well-informed about civil rights), but the population at large. And this population doesn't get any special training about their rights, but picks it up from movies and similarly unreliable sources.
So, even though police's training tells them that they shouldn't behave in such a way, they are confident that there will be very little backlash against such behavior (except from the "weirdos", which are a tiny tiny minority), and so they'll do it despite their training.
In summary: yes, we can blame police for this behavior. But we (the better informed people) should blame the movies as well.