My chiroprator cc slip had a "tip" space, like a restaurant!

OK I have never posted anywhere on DU except as responses, but this little thing that happened to me has me so annoyed I just want opinions.

I had a serious neck/upper back injury a few years ago, and have had trouble finding a really good chiropractor. This week I was beset with a horrible back spasm out of nowhere, I couldn't even lift my hands over my head, basically bedridden. So my husband sent me a list of chiros on our plan, and I went to one. She was young (28ish?) and the place also hosted massage therapists and accupuncturists. I know massage therapists traditionally get tips (don't really know why) but anyway, so this lady does a great job on my back, I am loving life again. My copay is only $15, she prints me out the credit card slip to sign and there is a space for the tip, and then I am supposed to write in the total amount. WTF is this? If I go to a doctor for a broken leg there is no tip area on the bill, it just really pissed me off. I did not write an amount, just signed the slip. I was really offended by the whole thing.

Now I would like to hear opinions about this. This was a real medical problem, not some juju crap and not some cushy massage or manicure or whatever. Do any of you tip a chiropractor? Would you tip a physical therapist? I mean where do we draw the line?