Maybe most people don't understand why it is insulting? Those that do, probably aren't working for near minimum wage. The degree of insult is directly related to how much you value yourself. Or, more specifically, knowing how much you are worth.
I think I can prove this point. Most of you, at some point, would find a bonus insulting. Where that point falls, varies, but it is there. Is is a 1% bonus? Is it a .05% bonus? a $10 bonus? Where is that point?
If you have no point of insult, I find that sad. You are worth more than that. You all are.
I am biased. I tip 20%-50% on a regular basis. I see right though 5% tips/bonus as the insult they are. But, hey, 5% is 5%. We are all worth at least that.