According to Merriam-Webster, brainwashing is:
1. a forcible indoctrination to induce someone to give up basic political, social, or religious beliefs and attitudes and to accept contrasting regimented ideas.
I don't consider that the WTS uses force, though there are some blurry lines there. But the second definition is:
2. persuasion by propaganda or salesmanship.
In which case, yes-- the WTS brainwashes people. At the same time, that definition is a bit broad. It's not unusual for us to be persuaded through the use of propaganda or salesmanship, so we're all brainwashed in some way or other.