Yeah, I noticed as well when I decided to check for purposes of this thread's discussion that it's very, very hard to find the $30/mo plan, and I'll bet that Joe Average would never be able to find it if he didn't know to look for it and have a lot of patience to do so. I don't understand why T-Mo won't let you just buy blocks of minutes to add to a pre-paid plan; there's no accounting reason I can think of, such as iOS for iPod Touch costing you money for updates, but iOS for iPhone does not, due to accounting practice arrangements established in accord with Sarbanes-Oxley.
*shrugs*
And as for the $30 plan, I think it's just a little less profitable for them than the other plans.