Why are cycle lanes not always safe?

We "londoners" are very used to crap cycle lanes all over the place. Cycling instructors do not recommend cycling on them unless there are wide enough to travel comfortably. In fact, most of the time they are just an indication for cars to be aware of cycles. This great video from New York not only shows why cycles lanes are sometimes unsafe, but also that you can get fined for not using them!



A few good examples in the UK below (from Guardian crap cycle lanes):