Haha, I think that's totally reasonable of them. As a consumer I would appreciate a notification informing me that I couldn't tip, no matter how much I thought they deserved it.

Edit: I wanted to add that there are a lot of places we're not expected to tip at whose employees deserve it just as much as the ones working at the places we are expected to tip at. Tipping culture may be out of control, but a lot of times people deserve a tip for providing exceptional service. That's our way of validating them and saying, "you're going to make it."