I think, for me, and many parents, it is the simple act of "forbidding" parents from watching practice. Discouraging is fine, although our gym doesn't even do that. I won't go to a school, doctor, dentist, etc that forbids me from observing, so why would I go to a gym, where my son spends 16+ hours a week training?
I think it is in the way the policy is carried out. Most of the parents at the gym do not sit and watch. They may stop to watch a rotation, or glance out to see what their kids are doing. Most of us are there because it does not make sense for us to drive back and forth. And parents tire of watching endlessly.
Forbidding something only makes that something seem mysterious and worrying. And while there are not any bad problems in most gyms, all it takes is one to get parents up in arms about what is going on behind closed doors.
So, while I can't stand to watch too much of practice, I am often at the gym. I am reading, doing work, or chatting with other parents. We are booking hotels for meets, or helping get things ready for a home meet. I would not attend a gym where my attendance there was "forbidden."