There are two different issues here - low pay and being treated badly. The notion of banding together and boycotting assumes that all shoppers believe the pay is too low. The choice is individual, whether you're an employee or an indpendent contractor. $4 or $5 shops are ideal for some, or they would not be offered/shopped/paid. That's free enterprise. Some shoppers want to work their way up, others don't. Some stick with it, but not all. Attrition is constant. Entry level positions are essential.
My criteria is different from yours - whoever you are. I did the $1.75 phone shops one month; it worked for me. Others find that amount ridiculous, even insulting. By accepting that shop, or any low paying shop, it does let the MSP continue to offer it at that price - the market will bear it. There are shoppers who are more suited for some shops than others, and not just because of what the payment is. The pay is probably the first thing I look at, but it's not the only thing.
Being treated badly is a different matter. A rejected shop is tough to swallow, though that is not necessarily being treated badly. If there are legitimate reasons provided, whether the shopper agrees with them or not, it's not a personal attack on the shopper. I've done 800+ shops and have had 3 rejected. Hated it, moaned and groaned. I had a choice to terminate with the MSP, or not, and the MSP had a choice to assign me another shop, or not. That's business.