No, it isn't. In practice no human will care whether you push the button. The social understanding is that it's fine to just take the free candy. In fact, the employees probably don't want to hear the ad again, so it is an ethical imperative to not push the button and subject them to that.

There is probably some human who sold the ad space to somebody, and who is monitoring how many button presses there are. And they will probably put pressure on the supermarket to make sure customers are reminded that they have to push the button if they want candy. Sure, the employees are probably sick of the ad, but the people who don't have to hear the ad don't care about them :)