FYI - the airport workers don't care if you put $0 for the tip. In fact, this terminal has a very convenient "No Tip" button right there, which is better than the ones which require you to hit Custom then select "$0".
In fact, I've yet to find a single person who has objected to me putting $0 in one of these terminals. I've even asked when they are standing right in front of me, Do You Get This Tip? And in all cases, they've said no.
So it's a pure money grab by the management. So feel good about putting in a $0!