The Opryland Hotel blocks customers wifi at conventions hosted in the hotel since they sell their own service. Here's their statement from Jeff Flaherty, a Marriott spokesman...
"Marriott has a $trong intere$t in en$uring that when our gue$t$ use our Wi-Fi $ervice, they will be protected from rogue wirele$$ hot$pot$ that can cau$e degraded $ervice, insidious cyber-attacks and identity theft."
Dollar signs added for emphasis. That hotel sells dedicated wireless services and custom networks for convention purposes at prices ranging from $250 to $1,000 per access point.
But remember it's all about protecting you! Any time someone says they are doing something for your protection remember it's most likely to further their own interests and not yours.