I do think WDW are at fault here.
Yes, I knew Florida was gator-central. I have also seen alligator warning signs on Disney, but away from the main resorts - more in the waterway/swamp type areas (hard to describe, but in between all the properly developed bits).
There aren't signs near the main hotel beaches warning against alligators. And, given WDW knew they were there, and knew they were dealing with them constantly, there should have been.
WDW have been incredibly lucky that there haven't been more accidents, or that it has taken this long for an accident to happen. Because luck is all it is.
Hotel guests are encouraged onto those beaches, to watch movies, to watch the fireworks. They are beautiful places, with chair swings, gorgeous sand, no indication whatsoever that there is dangerous wildlife in the waters, and no way of gaining that information. You are not to,d on check in (we have stayed on property several times, it's never been mentioned); it isn't contained in the resort info in the rooms (trust me, I've read it all several times); it isn't mentioned on the info channel on the tv.
There are no swimming signs, but in the absence of any other info, and given that water sports are encouraged on the lakes, and that the beaches are promoted heavily, it is reasonable for guests to assume that is down to lack of lifeguard/deep water/not sanitised to pool standards. That is the assumption I've always made when my gaze passes the No Swimming signs.
Add to that the fact that everything at WDW is micromanaged to a ridiculous degree, a fact of which they are very proud, to ensure an as near to perfect holiday as is possible, and you are left with the fact that it is entirely reasonable for guests to assume that the shoreline of a beach where families with young children are encouraged to gather, after dark, is safe.
And it is pure luck, from the sound of it, that this didn't happen years ago. The fact that there hasn't been an accident for many years doesn't mean that WDW have not been negligent in their duties in not informing guests of the risks.