The police can get them but they are told not to.
When I talked to an officer here once about cyclists going counter flow, running red lights and stop signs, using cell phones, riding on the sidewalk, riding at night with no lights or even reflectors, riding recklessly why they don't ticket them and his answer was that it's not worth the hassle because the courts haven't increased the fine on cycling tickets past $5 and it would cost the city a great deal more than that to process the ticket, so they're told not to bother ticketing cyclists.
The other weird thing he said was it makes accident reporting easier if a cyclist for example if riding at night with no reflectors or lights, and even if riding legally in their lane, or stopped for a stop sign then went, that if a car hits them and would have normally been at fault during the day it's automatically the fault of the cyclist at night because they were riding without legally required equipment and thus impaired the ability of the motorist to see them.
Fortunately some cities are starting to wake up and have raised their fines on cyclists to match those of vehicle fines and police are now pursuing bad cyclists, but so far only a handful of cities are doing this but it's on the increase and hopefully on the fast track. Tickets for bad cyclists should have been going on a long time ago, if law enforcement had been doing this all along we wouldn't have anywhere near the number of bad cyclists, more cyclists would be alive, and more people would probably commute by bike though that would probably would not be a significant change from current figures.
However the flip side also needs to be enforced too, and so far at least where I live the police haven't done their job at enforcing the 3 foot law. I had a car the other day pass with about 6 inches of space from me with a police car right behind them and the cop never did anything. Most motorists are being respectful to the 3 foot law, again at least where I live, but like anything we get the occasional law breaker.