You bring up a good point, but there is an equally good response: The cost of selling on Amazon is higher than on other platforms, for example Ebay. Amazon has forced free returns that are easily and often abused (and generally, more pro-buyer policies across the board), higher sales fees, ridiculously pro-accuser anti-counterfeit policies (often abused by competitors), highly inconsistent (buggy) within-Amazon pricing controls, inconsistent "gating"... I could go on and on. You wouldn't believe what a mess their third party seller system is unless you've tried it or worked for Amazon (I've done both). Usually you can get problems resolved by a phone call, or several phone calls, but often it is just too time consuming to justify. Just yesterday my partner complained that they lost her FBA inventory. There might be a way to prove it and get compensated, but it probably won't be worth the time cost. Customer service reps want to help, but I've heard they are overworked and poorly incentivized due to Amazon's naive productivity tracking of them (not verified; I didn't work in Amazon retail).
Now you might be thinking, why would any third party seller put up with this? There is one answer: volume. For two reasons, you can make more sales on Amazon than any other platform: their market dominance, and FBA.
None of what I've said is controversial amongst third party sellers. Despite all the frustration, most of them, including my partner, are willing to eat shit as long as the sales are good.
The solution most third party sellers dream of is not regulation, but rather a competitor with a better (i.e. less buggy and frustrating) sales platform.