It's not illegal in the States, just sold under a different official model designation with changes to comply with US safety & emissions regulations, i.e., different bumpers and catalysts on the M1 derived M88/S38 engine.
The E24 M6 (sold in Europe as M635CSi) was part of the original holy trinity (E24 M6, E28 M5, E30 M3) that spawned from the seeds of the M1.
Colloquially, the E24 M6 is affectionately called the "sharknose M6"
Its surprising how underrated the E24 is, even among BMW fans.
Original launch promotional video featuring F1 legend Nelson Piquet: