T-Mobile divested (sold off) their prepaid business as part of preparations for the Sprint merger. They, and the other three major carriers, are getting out of the cheapo prepaid business.
There is no need to use T-Mobile. You can use a MVNO that offers a cheap per-minute monthly plan. Remember: you are essentially wasting the service provider's time and money by porting in, then out; therefore, many service providers have restrictions, such as requiring that you maintain service for a month first. You'll have to do some research on this.