They never learn... Some time ago (early/mid 90s I think) there was a particular problem with pickpockets in the Kings Cross area so an awareness campaign was started with similar posters. The problem got worse. Why? Often people walking past the posters would, consciously or otherwise, check the pockets their wallet or other valuables were in - this helped improve an observant pick-pocket's hit rate.

I know what I'm going to do in Chicago! I'll buy an ad like that near one of the CTA platforms and stand around it and as people come by and try to scan it, I'll pick their wallets.