I'm not sure where you are reading/seeing this, but I've never seen it happen, nor have I ever heard of it happening, and don't really believe it *would* happen, for two big reasons: (wow, that was a run-on sentence!)
--It's impossible to know what is "official" Disney vs homemade vs Etsy vs badly-executed-cheaply-made-officially-licensed (think Dollar Tree washcloth, for example).
--There's no way all the stores on Harbor would be selling Disney shirts (though, they don't usually say "Disney") if people wearing them were getting turned away at the gates. Can you imagine the riots it would cause?
In short, you won't get turned away for wearing a non-licensed shirt, unless you are (actively) marketing them inside the park. So, don't wear a sign that says, "Ask me about my Unofficial Underground Mickey Mouse Shirt Shop!" and you should be good!
