Well, not exactly.
It depends on the license the original code is under.
Many open source licenses require you to use the same license when you use parts of whatever their protecting.
Correct me if I’m wrong, but I don’t think sponge requires you to license your pluigin with any specific license. Only the Sponge API is specificaly under MIT.
PS: Besides that, most Bukkit plugins will simply not work with sponge without some tweaks by the developer.