Many of the smaller bolt heads on MTB components tend to be Torx these days, especially for certain brands. For example, of the components I own, the SRAM mtb derailleurs, brake levers, brake rotors, shifters, etc use Torx-headed bolts, yet all the SRAM road components I own are strictly hex keyed. Why they don't stick to one style- preferably Torx- I don't know. I do know that Torx provides more "bite" which results in a bolt head that is less likely to strip, especially in smaller sizes. This makes a lot of sense, especially where higher torque is required such as brake rotor bolts which typically require a higher torque specification in comparison to similarly sized hardware.
That said, there would be no real technical or performance reason to have only one Torx bolt per rotor. I know that many cars with alloy wheels use a single lug nut with a special head per wheel and normal lug nuts on all the rest of the bolts for theft protection. Perhaps this is supposed to be something analogous to that. Alternatively, since you stated that this was a BSO (or Bike-Shaped Object, usually referring to a low quality or department store bike), it's possible that a) a single Torx bolt is used to discourage low skilled workers that are assembling the bikes from removing the rotors, especially if they are of differing sizes, or b) this is simply what they had laying around and the single count on each rotor is a coincidence.
Bottom line, you would not find this as a "feature" of disc brakes or bikes equipped with them of any real quality.
