My experience with Vegas is that the rules are always different with what hotels get away with. Some of this is good...usually you get free parking.
But hotels being hotels, they've all never met a fee they didn't like, so I presume this business with charging for incoming packages is a popular little local scam. I've never encountered this particular one at any Marriott, but if the company allows managers' discretion....and they can get away with it...there's your answer as to why it exists.

I've also found in my experience that if you're a good enough customer (i.e. Platinum), and you complain enough, they'll make a lot of this ridiculous stuff disappear.

My favorite one of these, BTW, was at the San Jose Fairmont. About ten years ago I was there for a meeting, and an associate of mine had forgotten a telephone cord to connect his laptop to dial-up internet. No problem! The hotel gladly loaned him one. And then charged him $35 when he returned it!!!

(He screamed....our company had a corporate account with the hotel...and got the charge removed. But from that, we also all learned to just go up the street to the local Radio Shack and buy the same cord for $2.98).