The problem is that pure chlorine is a gas and very dangerous. It has to be bonded with something else to get it into a safer form. The "something else" is usually something you don't want in your pool. In the case of bleach, the "something else" is sodium which is probably the least troublesome option of what's out there. The tablets you saw have to have something else in them besides chlorine. Try to find an MSDS sheet on them and that will tell the story.

Peter