It is their job. They are not private consultants. Like others have said, if the pay is too low, they should focus on that. Genius bar service is a free perk for customers. There is no payment for most of those interactions so adding a tip would create a whole new payment flow and make the whole experience much less customer centric. It would be a serious detriment to the Apple Store experience for customers. They must know there is zero chance Apple would agree to this, so it must be a toss-in as something the negotiators can 'give up' in a fake show of good faith.