I feel so badly for these parents. What a horrible thing to experience.
We just came home from Disneyworld on June 4th. We stayed at Sabal Palms and Saratoga Springs and no signs were posted at either resort that I noticed. I told my kids when we first arrived at Sabal Palms (we had a ground level room that overlooked a lake and golf course) to watch for any gators while sitting in our screened in back patio. They (including my husband) didn't believe me that alligators could be in the water.
We rode the monorail over to the Polynesian from the Magic Kingdom. The beaches created here are obviously man made and I can see where a family would assume they are safe. I heard Alligators are everywhere but I also would assume Disney would be monitoring and ensuring designated play areas are safe. Chairs, beach toys, movie nights, etc all make you think this is an area to be playing in. Where I'm from "no swimming" means no lifeguard on duty or no submerged swimming due to some other reason (steep drop off, rocks, strong currents). It doesn't mean no touching the water or wading in it.
Just a horrible situation, very sad. I'm sure many things will now change at these resorts to prevent this from ever happening again.