The fine was for disrupting wifi hotspots in the convention space at a single Marriott, but Marriott then petitioned the FCC to allow them to do the same for any guest on their property, including properties without convention spaces[1]. The FCC turned them down.

(I've amended my original post to include a link from the conclusion of the whole affair)

[1] http://apps.fcc.gov/ecfs/document/view?id=60000986872