Bell Gardens Massage Lianye Yang Therapy Forced my sister and I to leave a tip Bell Gardens California

So my sister and I went to get a massage and the Asian ladies forced us to a pay a minimum of $5 tip each. My sister was done 15 minutes done before I was and gave the lady a $3 tip. After she had already given the lady a $3 tip, she stood there with her hand out demanding more money. When my sister asked her what she wanted, the lady pulled out a hidden sign that said minimum tip charge $5 for foot massage and the manager forced my sister to pay the $5. About 15 minutes later, I too gave the lady a $3 tip and she stood there staring at me with her hand out, until she called the manager. The manager once again pulled out the hidden sign that said minimum tip charge $5 for foot massage.

After I explained to her that it is not required to leave a tip and the sign says "foot massage", which I did not get (I got a body massage), the manager stated that I should leave $10 or $15 instead of $5. My sister and I once again told her we are not required to leave a tip and that I did not get a "foot massage" like the hidden sign stated. After a couple minutes of frustration the manager demanded that I leave at least the $5. I decided to leave her the $5, which I was forced too and as I did and the other lady who had given me the massage gasped to the idea that I was only leaving $5 and furiously said "$5" and walked to the back. Am I really required to leave a tip, even when they are acting like this? What should/could I do or have done about this? I am really upset.