Hussar
Legend
The shield bash from Shield Master feat uses your bonus action and you can only use it if you are going to use or have used the Attack action that round. So, they aren't giving up an attack for this.
Now, the Shove action which anyone can use, does use an Attack from the Attack action in place of an actual attack.
Meh, you're still dropping your bonus action and it did cost you a feat to be able to do that. Considering the alternative to a possible opportunity to gain advantage versus, say, Great Weapon Mastery or Polearm Master, or several other feats, I'm thinking that the power gain is minor.