Looks like the hotels are claiming this is security and performance related.
Mobile hotspots can be used to “launch an attack against [a hotel] operator’s network or threaten its guests’ privacy” by gaining access to credit card numbers or other personal data, the hotel group said in its petition.
Maybe. If the mobile hotspot is called "Marriot Free Wi-Fi" but is operated by someone collecting information on anyone who connects. Then again, this could happen anywhere. This is why you don't connect to strange wi-fi networks. If you must connect to your hotel's wi-fi network, make sure you're connecting to the right one, not just one with the same name. The solution here is guest education (post signs about which Wi-Fi network to connect to, etc), not running a jammer to block everyone else's Wi-Fi signals.
Multiple outside Wi-Fi hotspots operating in a meeting room or convention center can hurt the performance of a hotel’s Wi-Fi network, the group said.
My off-the-shelf router handles multiple wi-fi networks just fine. I connect to my Wi-Fi and my performance isn't degraded because my neighbors run Wi-Fi networks of their own. A hotel should be able to invest in the infrastructure to provide their own Wi-Fi that will work regardless of whether or not I turn my phone's Wi-Fi hotspot on.
The "security" and "performance" claims are garbage. The real reason is that they want to be able to sell you their Wi-Fi service for a ton of cash and it's hard to do this when you can bring your own Wi-Fi network in with you. As gurps_npc pointed out, if we let them do this, how long until they block all cell phone signals because it interferes with the "security and performance" of their phone system?
Educate? The users? Asking users to only connect to "The REAL Marriott wifi" is all kinds of nuts. You might as well issue them a 802.1x username/password since they are as likely to get all that shit right as they are to tell the difference between "Marriott" and "Marriot" and "Marriott Wifi" (and know which one is legitimate). Your best hope is that you are able to give them a unique WPA2 key that would fail when connecting to anything but the right AP. Even then you have to impress on the importance of actually putting the key in and not just connecting to whatever pops up and doesnt require a key, and since users follow the path of least resistance this option is bound to fail as well. A signed certificate for Wi-Fi SSIDs is hugely overdue, and the fact that we have gone through so many iterations (b, g, a, n, ac) and haven't even taken a crack at it is very disappointing.
While I don't think Marriott, etc should be allowed to do this (since it is clearly in violation of the ISM rules) it's sensible since it was clearly effective (otherwise they wouldn't have lost that judgement).