This is definitely true. If one car in a blocked off lane slows down to pull over, that will slow an entire line of traffic behind initiall, and continue to do so, as about 1 jackass out of every 100 cars will brake to watch what is going on and slow down the whole lineup.

The funniest incident I have ever seen though was when in my brother's vehicle. We were in the far left (non-HOV) lane. An officer had someone pulled over up ahead, so this one woman by herself in the HOV lane panicked, and cut across the double-yellow lines without signalling, cutting us off in the process (4 violations at once). My brother honked to draw attention- the police officer saw us, then went to pull her over

The one place I have seen HOV lane violations enforced, however, is on onramps, where it's possible to bypass the traffic meter signal in the HOV lanes. This doesn't cause any backups as there is a lot of right-hand shoulder space to pull onto. A friend of mine actually cut one of those and ended up with a ~$480 citation, along with about 10 other peopel that day. I notice the police like to pick a few days a year at random to enforce things like this, although the chance of getting pulled over is small, even years worth of habitual time-shaving are not worth $480.

What would be interesting would be to see some sort of traffic camera to cite violators. The problem with that however is it would be determining who the violators are by sight without actually pulling them over, as it could be argued they had a child in the backseat, or their passenger bent over to pick something up etc.