Maybe she was afraid you might be LE. Did she ask questions like "Have you been here before", and if you say yes, then "Who did you see?", or "Is this your day off from work?"
Sometimes if they don't know me they will ask where I want to be massaged at the end, and then ask me to take their hand and actually put it on that place. (I wonder if that is an absolute defence against LE?)
I haven't been to any spas, Asian or otherwise, that require my underwear to stay on, so either she was afraid you are LE, or they are an ultra-straight RMT establishment. (I've been to many spas that were strictly no-extras, in which case they just put a towel over my naked butt and kept it in that area when I turned over.)
In any case, I always ask before I try to touch, even when it is obvious that the spa offers extras.
There have been some places I was sure were totally straight that, when I asked, I got hand release and once in a while being able to touch their butt or back only over their clothes. But again, I asked first what was allowed, and they were not shy in saying what was and what was not available.
Another hint as to what is possible is when, near the end of the massage, they do very light touch massage, almost tickling, with some attention to your butt and scrotum underside before asking you to turn over. If that doesn't happen, I assume there isn't much extra available. Nonetheless I still ask, and the odd time get very limited extras.
Anyways, if you're interested in going back, maybe they'll be more comfortable having seen you once before, and may offer extras, or be receptive to your verbal requests.
Also, a slightly oversize tip after the first or second massage, say $20 even though you didn't get any extras, may pave the way to a more interesting session next time.