Yep, dburg30 is absolutely right. SW isn't considered to be a 'participating airline' officially yet. But, you can check your luggage at POP and the AllStars now. But, again, it's not official, and could possiby stop at any moment.
But...I doubt they are going to stop it. After the trial period at POP, they moved on to the AllStars....to see how it worked there. Seems to be going okay. If that works out, they will probably head to other resorts, and eventually become a 'participating' airline.