The FCC has no place setting fines based on the total size of a corporation. It seems rather proportionate to me, given that they were attempting to profit from willful interference, and that's in the ballpark of how much they probably made. I doubt Marriott, or any other major US hotel chain, will ever attempt this again. Seems to me the punishment worked, without having to get dragged into courts, appeals, and legislation if they had, for example, fined them $1 billion.
Even if I'm off by two full orders of magnitude it'd still be $960,000, and it's possible I'm undercutting the truth.
Also, don't forget the cost of doing compliance audits across all of their properties for the next three years, which is not going to be cheap either.