I think there used to be lore that there was some kind of secret police that literally hunt down and force the treatment onto anyone who don't want to take it for whatever reason, but that was deemed too fucked up and was dropped.
My interpretation is that taking the treatment is more of a cultural mandate than an outright legal one, what with how much of a role it has played in the planets history. But that role has long since passed, and the treatment is arguably holding the people of New Texas back at this point, with how random and often... intense, its effects can be giving the people a reputation as being easy and/or airheaded, like what we hear from Kimber. So now Big T is trying to put in motion things that will phase put the treatment, or at least stabilize it so that the New Texans can loose the bad reputation they've gotten.
Unfortunately, things aren't there yet, and many New Texans still expect that to be a New Texan, you need to be Treated, which is why it's part of the immigration process.
I really would like to see someone write a character who is similar to Rhea, but at like, a younger stage in their life. Maybe not outright rejecting the treatment, but just... scared of it. Worried that it may change them into a different person, and as a result decided to run away.
.....................................
oh my god.