I actually had a conversation with a Guest Relations CM recently in which this came up.
She expressed her regret about them, confirmed they were not allowed and they are constantly removing them from the parks.
She then lamented that the people up front (both security at bag check and the people manning the turnstyles) need to do their job a little better.
Her words, not mine, just thought it was amusing to see how the people inside get frustrated at the people out front not doing their jobs. That is probably the reason more and more of these are seen. The CM's inside figure if the ones that are supposed to catch them aren't going to do their jobs, why should they.
Not saying I like, agree, or understand that attitude, just saying it's there.