Hotels and convention centers offer many free amenities, but lately it’s becoming more common to find many are charging for Wi-Fi connections. Now some venues are coming under fire from federal regulators, who say the companies have maliciously blocked consumers’ attempts to use their free personal W-Fi hotspots.

Earlier this week, the Federal Communications Commission (FCC) announced it has fined the contractor for the Baltimore Convention Center $714,000 for blocking Internet access to convention goers. The agency is also contemplating levying a $25,000 fine against Hilton Hotels for hindering an investigation into its Wi-Fi practices, according to PC Magazine.

The FCC says that blocking Wi-Fi access violates Section 333 of the Communications Act of 1934,  which makes “willful or malicious interference” with radio transmissions illegal.

With shrinking margins, hotels and convention centers are looking for new ways to maintain revenue, and fee-based wireless access can be a lucrative service. But federal authorities say that blocking established Wi-Fi networks for profit constitutes illegal interference with radio networks.

The FCC complaint against the Baltimore Convention Center operator MC Dean alleges that the company used “auto-blocking” to jam users’ access to their personal Wi-Fi hotspots, leaving the company’s fee-based Wi-Fi as the only available network. The company charges convention goers up to $1,095 for an Internet connection.

In November 2014, FCC commenced its investigation against Hilton, asking the chain in a letter about complaint the agency has received about Wi-Fi practices at several of its properties, which include Hilton, Conrad, DoubleTree, Embassy Suites, and Waldorf Astoria hotels.

But the agency says that, to date, the company has failed to provide information on most of the properties.

Last year the agency fined Marriott Hotels $600,000 for blocking Wi-Fi transmission at the chain’s Gaylord Opryland Hotel and Convention Center. According to the agency’s investigation, Marriott blocked guests’ free Wi-Fi hotspots, pushing them to the company’s network, which charges up to $1,000 for Internet access.