Two quick thoughts:

(1) I don't think that tipping is "altruism", but rather meeting a *special obligation*. I don't think it's "especially" morally regrettable when we use some of our resources to meet special obligations in a way that's impartially suboptimal. I would much prefer that people expanded their charity budget from out of their "personal luxuries" budget than from their "meeting special obligations" budget.

(2) Being suboptimal isn't sufficient to be "morally regrettable"; it depends what one otherwise would have done. Few people who stop tipping will use the savings to do more good. Most will just pocket the change. So it's morally preferable (rather than regrettable) that these people continue to tip.