It's called negotiating. Nothing says they cannot do it.
1)
Actually, the National Labor Relations Board (NLRB) says this is wrong.
2) You cannot use pay inducements
in this manner (as coercion or threat) while in negotiations.
3) I know!
4) I lost an unfair-labor-practices charge in an NLRB Administrative Hearing case when we did (almost) the same thing.
5) But, in our case, it was inadvertent
. . . we verbally announced to employees in plant-wide meetings an upcoming pay increase, but simply didn't post it until 2-weeks later
. . . we had gotten a new-huge contract and were rewarding the employees for quality work that helped get the new work
. . . we did not withhold the raise or threaten to hold it back or put any conditions upon the increase
. . . it was just a case of bad timing
. . . the judgment against us (and me) said we were trying to "influence" and/or "entice" employees
. . . with WDW, they are actually threatening to withhold publically announced cash unless the union settles