Why do I keep getting charged for individual songs if I subscribe to Apple Music??

Correct. The plus sign will add the track to your library through Apple Music. The store will have prices.

There should be no behavior where clicking add will charge you for the song.

You may want to contact support at getsupport.apple.com

Likely because you are purchasing songs, not just listening through your Apple Music subscription.

Navigate the to Apple Music section, not the iTunes Store to listen to, and search for, music. If you're in the iTunes Store and see a price, that purchases the song.