I'm familiar with USB keys in general from other software that uses them, but how would it be 'optional' on some SAM versions? Do they have some other alternative copy protection scheme in place? Is only a limited set of features available unless you insert the USB key? I ask because sometimes I max out my available USB ports, and would rather not bother adding a hub unless necessary.
It would use a hash key on the installed version. By that I mean "like windows" there is a validation code you have to enter. The "optional" probably means that you could get a dongle for it if you want one. Why would you want one? The advantage of that is if you have multiple computers and you want to run Samplitude on all of them. You take the dongle with you and use the dongle with the local version. It won't work without it, but it would allow you to use the software whereever you are.
Different users have different needs. Some just need it on one computer, others need portability, and some need to run from a network. They seem to have addressed all of these scenarios.
Users complained big time about the dongle at first. So they went with no dongle on the lesser versions. It's cheaper to implement for them per user as well. However, they protect themselves against pirating moreso on the multi-featured versions.