Idea

I am against this idea.

I made the mistake of uploading to a PoD site that has a similar policy, and all my designs were stolen within minutes by what I assume was a bot that simply mass-downloads new designs and reposts them to other sites. Anyways, more reasons this is a bad idea:

1. It would deny honest designers the ability to build multiple brands around different niches.
2. It would paint a target on the back of more successful merch sellers, making it easy to identify their accounts, inviting retribution from jealous and anti-competitive sellers.
3. It would shine a giant spotlight on your market research, allowing the most cut throat sellers to pilfer your ideas as soon as you publish them. Say goodbye to quiet profitable niches and hello to a flood of knock-offs. Aggressive sellers will use web bots to stalk successful accounts, automatically downloading any new designs published, which they will send out to their designers to copy. Unethical sellers, meanwhile, will simply repost your designs to other PoD sites as-is. Instead of ending the copycat plague, you'll have just made it MUCH easier for them to sell your shirts on non-Amazon sites.
4. Do you want a website where people can look up the total royalties of every single merch designer? Because within months of this policy being implemented, someone will design an application that collects BSR and sales information of every single merch account. Your sales numbers will be public knowledge and that knowledge will be exploited by unethical people. People will create "top 1000 merch sellers!" type sites, and the top sellers will see a huge increase in people knocking off their designs.

Any policy that penalizes honest designers in order to tackle the abuse of underhanded sellers is a bad policy. There are way better ways of dealing with pixel thieves.

This thing is starting to smell like an inside job.

people need to be able to protect brands / have multiple brands of differentiated shirts