For most sims, the "dongle" (aka: interface) is their copy protection to prevent unauthorized copies from floating too wildly. Reflex, AFPD, Phoenix, G3.x, FS One... every major commercial sim has some sort of dongle in use (I should know, I've got 5 of them right in front of me.)

The freeware, shareware or "hobby programmer" products probably don't use any form of hardware protection.

-Case