I believe the poet is using a counter-intuitive, paradoxical construction to convey a poetic sense to the reader. Shackles are used to chain someone and to restrict them from moving freely. But the poet has reverted the meaning by saying that these shackles (the shackles of love, presumably) are actually making them feel free. They probably want to say that they feel free because they have let go and have lost control. In other words, they have stopped being worried about the consequences of their actions and they have stopped worrying.

It's just a song. It's a lyrical technique. So typically "shackles" chain one and prevent one from being free. But to make the song more interesting or surprising or something, the writer has reversed that. I think songs are horrible to study languages by. Maybe pronunciation, tone, to become familiar with the language... but the lyrics themselves... who knows where the author is coming from and if it's a mainstream song, it's probably just a gimmick or a love song to get people to buy the album.