Following on from request for clarification, the FCC has now made it perfectly clear on what's prohibited.

Persons or Businesses Causing Intentional Interference to Wi-Fi Hot Spots
Are Subject to Enforcement ActionU.S. F.C.C. Clarifies "WiFi Blocking Prohibited" [fcc.gov]

No hotel, convention center, or other commercial establishment or the network operator providing services at such establishments may intentionally block or disrupt personal Wi-Fi hot spots on such premises, including as part of an effort to force consumers to purchase access to the property owner’s Wi-Fi network. Such action is illegal and violations could lead to the assessment of substantial monetary penalties.

Some of you may remember this case from last year when the Marriott Hotel was blocking users.
Marriott to pay FCC $600,000 For Blocking Wi-Fi [webmasterworld.com]