I don't really think you have an arguement, you created a listing (for your own benefit) for a brand you do not own or have any relationship with but somehow consider yourself entitled to have the listing to yourself and/or complain if other sellers join. You haven't created the product or the brand, just an Amazon catalogue entry.

Amazon only create listings for their own products so the vast majority of listings are created by other sellers some of which you may have jumped on at some point.

We all create listings when the product isn't in the catalogue or jump on the existing listing if it does, we can't do anything else as the EAN is locked to that listing. Sometimes we can't list things because the catalogue entry is so bad or misbranded but that's another story.