There probably is a START instruction internally, but it won't take action against a number for which there has been a previous STOP. So UNSTOP acts like FORCE START.
If the process is releasing a STOP (removing or soft-deleting a STOP instruction logged in the DB or some such) then whoever worked on it initially may just have not thought beyond that (especially if English wasn't their first language so unstop might not have sounded any stranger to them than restart). Once something like that is written down and others have seen it, it tends to stick.
Of course it could hav ebeen done by someone like me, who is know to give things technically-correct-but-odd names deliberately…
