Calling the Shinsengumi a police force is like calling the KGB a bunch of cartographers. The Shinsengumi were basically a paramilitary hit squad that assassinated people that they believed to be anti-Shogunate, and later fought with the pro-Shogunate forces during the Boshin War.

A better way to frame the Shinsengumi in this context would be like how we view John Wilkes Booth. Even though his cause was doomed, the Lincoln assassination is still often invoked in American culture and fiction. The idea of a man who would assassinate people just because he truly believes in a political cause is a source of interest for people. So the Shinsengumi were featured in several "jidaigeki" films, or "period films."

More recently, NHK produced "Shinsengumi!," a taiga drama. NHK, the Japanese national TV network, usually produces a long period drama (the taiga drama) every year, based off of a Japanese historical event or person. In this case, they featured the Shinsengumi.