The Tempest is obsessed with the concept of imprisonment – both literal and figurative. Prospero and Miranda are forced to live in exile on a remote island, where Prospero enslaves the island's only native inhabitant (Caliban) and forces Ariel to do all of his bidding. The theme continues into the epilogue where Shakespeare suggests that, during the performance of a play, actors and playwrights are held captive by powerful audiences who may or may not approve of the artists' work.
Because Prospero forces Ariel to serve him, he is no better than the witch Sycorax, who imprisoned Ariel in a pine tree before Prospero came along and "rescued" the sprite.
Prospero is not free because he is subject to his own desire for justice; he is a slave to the past wrongs done to him.