MOTM

This is a question in the most general sense but, of course, it can 
be applied to some MOTM stuff (and Oakley, and FutureRetro, and 
Frostwave, and Studio Electronics and and and) too...

How do people get away with manufacturing - or if not manufacturing 
providing schematics for - filter, oscillator, effects, etc. clones?

Are things like filter designs patent-able?  Have the patents to 
filters like the MS-20, TB-303, etc. run out and are therefore open 
game?  Do the manufacturers not pursue it since these designs are no 
longer in production?

I'm sure $$ is a limiting factor in pursuing these things and so a 
boutique designer might decide it's not worth the hassle but I would 
think cost for someone like Korg, Roland, Yamaha wouldn't be a factor 
(to an extent).

Just curious.

JP