I can imagine some harm comes from this - e.g. an employee is mistaken for a model, or perhaps an employee is never mistaken for a model, but the magnitude of the harm doesn't seem so severe, and seems comparable to the benefit - the party is more fun.
I know why people have an intuitive dislike for this, it calls to mind paying for sex, sounds pathetic, and feels possibly misogynistic, but logically, I can't defend any of those positions myself.
This doesn't seem that complicated to me. I am not sure how I could possibly make what I am saying any clearer.
As a possible diagnosis of the problem you are having, if you believe my first sentence says or is equivalent to "I don't have a problem with this", you have a fairly common reading comprehension problem where you read what you expect to have read, rather than what was actually said, and the problem you are having with what I wrote is your difficulty in fitting such a short point into your framework of what you expect to have read. I suggest spending some more time learning how to read what is there, rather than what you think is there, as you are missing out on one of the primary benefits of reading. That's not the only possible problem you've having with understanding such short, simple sentences, but it is likely enough to be worth calling out.
My point is that employees should not be subjected to employers hiring models to attend company events.