Most casinos named after people (real or imagined) use the possessive form, as in Arizona Charlie’s, Binion’s, Harrah’s, O’Shea’s, and Terrible’s.
Though Fitzgeralds is named after Lincoln Fitzgerald, a Reno casino owner from the ’40s, it seems to have followed the lead of Caesars Palace, the most famous and original casino-without-an-apostrophe. The story goes that instead of implying that the hotel belonged to a single Caesar, developer Jay Sarno wanted to create the feeling that every guest was a Caesar.