My 2 cents:
I agree with you and I think it would be most helpful if UpWork helped employers be fair and responsible payers. I don't think it has to be a "rule" because some people may want to work for less to get more clients or they may want to gain experience. (i.e. I was an unpaid intern at one time)
However, it would be helpful to have some guidance for various country's cost of livings, minimum wage, employment laws, etc. I'm not an expert on the economy in various countries, but I'd like to be a fair employer. Some of our UpWork freelancers work very hard for us and we want to make sure we are paying a fair price for them (and for us).