IIRC walmart's BYO plan only works on phones bought AT walmart (though i can't get a straight answer on this- I'm getting yes and no from a dozen websites). and 100 minutes a month of talk time is ridiculously low- You can find totally unlimited plans at some of the smaller carriers (metroPCS?) for $45.
I didn't say a post paid plan was CHEAPER, I said that for most consumers, selecting a post paid plan that meets their pricing AND coverage needs is BETTER, since it's not likely most consumers are going to be jumping back and forth between AT&T and Tmobile- which is the primary benefit of an unlocked phone.
This is because (as we know) speeds and coverage between the big four varies a lot from region to region. An unlocked GSM phone that works on AT&T and Tmobile isn't going to do me a whole lot of good if Tmobile service near my house is terrible. It becomes an expensive AT&T phone- I might as well have just bought it subsidized from AT&T in the first place. Or Maybe verizon offers LTE where i am, and AT&T doesn't? If it were possible to cart the phone between all large carriers and get all the benefits that would be one thing, but there simply aren't enough carriers that support GSM in the states to make an unlocked phone worth the increased upfront cost.