Not always, unfortunately. My wife worked for J.C. Penney in the 70s and 80s. Her store was union, but not every store in the region was. She told me every time a new contract was negotiated, Penneys gave non-union stores a nickel more per hour than the union stores got.

I'm not a all anti-union, and it's possible the union stores had other benefits the non-union stores didn't. But blanket statements about union workers always having better pay aren't always true.