When the JV3 was new, I think 10 mb ethernet was all they had, USB was 1.1, not much better, where as firewire was (and still is) the fastest, except GB lan, which has just recently come out.
The Sp is just a souped up S, guess they didn't want to do too many modifications- If it works, dont f*** with it?
Even the new JV5 uses USB2, I don't think it has ethernet either.
Would make hooking them up easier though...