Because a family in Buckeye, AZ found rattlesnake in theirs.

Quote Originally Posted by Buckeye FD on Facebook
Apparently, 2 pool noodles were left outside of the pool up against their cinderblock wall. The next time they went to use the pool, the pool noodles were picked up and brought to the swimming pool. Out popped a rattlesnake. The snake did not attack, but was concerned about the pool noodles as there were a couple of young rattlesnakes who were still inside the pool noodle.

And here's a news story from Phoenix about this literal danger noodle. Not as informative as the Facebook post IMO, but it does give some context for those who don't live in Arizona.

Quote Originally Posted by Buckeye Fire Captain Tommy Taylor on Fox10 News
If you're going to be in the pool, and you have stuff that's stored outside overnight, might be a good idea to shake it. Might be a snake or scorpion in it
No thanks; I'll stay inside and crank up the AC.