Yes, it'll 'roam' on any network that ST has an agreement with. It will however only be for voice and texts. It'll also only happen when you get absolutely no Tmobile towers around to connect to.
In some ways you're right, it seems as if it's prepaid roaming, but it isn't. ST has made agreements with the two main GSM carriers, Tmo and ATT, and some of the regional ones too. Since there is not technically any 'native' network for ST, none of it is roaming, all of it is normal access for it.
None of that matters really since you basically get the largest network to use of anyone all while on prepaid.