Actually it is only prohibated as long as you use that very same protected object. By adding it inside Spiral Knights, it is possible to think up something completelly yourself, that just happens to 'look' like what Star Wars has.

Many games already have things like Light Sabres, but in those games Light Sabres do not come from Jedi or intergalactical clone wars, but are invented from this or that source that fits inside that game.

Example:
Spiral Knights Headquarter scientists have found a new way to support Crimsonite's natural energy to create an elemental sturdy and deadly beam with a limited length. This sword was pretty quickly known as 'Light sword' or Light Sabre' due to its appearance.

^
So in this example, it is not really that thing Jedis are using but a Crimsonite-driven energy module, that just happens to look similar to a Jedi's Sword from Star Wars. Jedis do not have Crimsonite and Jedis are definitelly not using Crimsonite in *their* Light Sabres; so it is a completelly different thing, copyright protection speaking.

Long story short: Just make something up and you can use pretty much everything.

This beeing said:
+1, I like that idea.