Well you've all seem as if you've been exposed to the bad apples which in turn, make the rest look just as bad. I can give insight as to why some (including myself) follow closely for brief (or extended) amount of time especially during night time when it's that much harder to see license plate numbers. 0's look like Q's, D's look like 0's, etc. If I don't have probable cause, I don't have it and I move on. No way I'd ever make something up to jeopardize public trust.
As far as changing lanes fast, every state/country is different so I can't speak for all of them, but the way I see it is that if you're doing no harm, I don't foresee an issue. Definitely don't recommended on busy roads, though. Just be careful on the roadways because it doesn't have to be you driving recklessly, but the inattentive, distracted momo beside you.
