Depending on the attitude I may not have tipped either. They should always ask you if you want a glass unless its a total dive bar.
That said common Etiquette is to tip about $1 per drink even its its just to open a beer- and yeah seems a bit ridiculous to me - but I expect it going in.
If its something fancy\complicated - like a high end cocktail - then more than $1.
Yes they do technically make less than minimum wage - but like I said earlier if they are not making a lot more they are working at the wrong place or are a bad bartender - I know very few bartenders who are not making 40-50 and hour. If they have a really bad week the business has to make up the difference and bring them up to minimum wage.
I've said this before but when I go to Universal for
HHN and am expected to tip on drinks from a vat that is not even served by a bar tender - than its really hard even for me to tip - they are just getting a pre made drink from a vat. I look at it it like is that $1 going to make a difference in my life? I'm already paying too much for a watered down drink from a vat so what's another $1.