When I was a teenager, I worked for Toys' R Us for awhile....
I can't remember the specifics, but there was a cashier who got slammed by a manager in front of a LOT of people for something that wasn't her fault... When they realized this and she was upset, they took her into the back office for a bit... She came back out... worked for a few more hours... and then got fired...
Turns out, they had given her a $2 an hour raise (Probably like a 30 or 40% raise for her) to keep her happy and keep her from complaining to higher ups I guess... Well, she came out and bragged to someone about her raise (which they told her not to talk about)... and someone got mad that they didn't make enough and complained.... she was toast...
Everywhere I've worked (in the US), it is definitely a firing offense to talk about your salary.... I don't know if everywhere cares, I was just always under the impression that you could be fired for that here.