It's not a difficult concept to understand and nothing to do with trying to police people's thought, but merely stopping the harassment that would otherwise take place. This particular incident would not have happened if the individual was praying somewhere else or not making a particular point about praying exactly where it would harass people using the clinic.

It's fine to have religious beliefs, but it's not fine to go around imposing those beliefs on others - that's how a lot of wars get started.