AT&T said that this will be for union employees. The unions have pay raises set in the contract so this won't affect if they get a raise or not. And with this being for union employees it is easier for AT&T to just give them a bonus than have to rework the contact to include the new raise.
This bonus is a good deal for the employees because the options are between a bonus or nothing, not a bonus or raise.
Here is an old contract as an example. (the pay charts start in appendix A)