[Zak]

I think they'd have to work a bit harder than that.

Even assuming they have continuous access to ebay, telco and mvno systems, they probably don't have continuous access to the computers of the individual reseller who's selling preloaded SIM cards. This is almost certainly one guy working out of his house.

No doubt, the NSA could hack that guy, but they'd have to do so deliberately. The connection necessary for mass surveillance is broken at this point.