Yeah, I don't see what the problem is. I backed it at the $150 tier, but I missed the survey to decide on whether to sell the backer exclusive content as DLC. Personally, I'd have said to go right ahead and sell it. I hate when I'm locked out of content in a game because I either couldn't preorder it, back it because of being short on money, or press an email confirmation quick enough.
I guess my mentality on kickstarters, and often the reason backers pay a premium, is that I'm backing this game so it exists. Without the backers, there's a very, very high chance this game wouldn't exist. A game in a genre I'm very much passionate about. So yeah, I paid a premium to help development come to fruition. I don't particularly feel that it gives me some kind of entitled exclusivity to content. Maybe it's because I'm a game designer myself, and work in the gaming industry, but I don't like the idea of keeping content from gamers that have paid for that content.
Sure, backers paid extra, so the game could reach stretch goals and provide content. If the motivation was just so I could get exclusive stuff that no one else could have so I could wave my epeen around online all smug and shit... that's just immature to me.
I backed Bloodstained because I desperately wanted an Igavania on consoles/pc/portable. I also wanted more gamers to have a chance to experience an authentic Igavania title (no offense to the myriad of indie titles clearly inspired by his work in the genre and were some pretty amazing Metroidvanias as well). I couldn't give a rats ass about non-backers being able to buy the Iga Back Pack. I'm just happy the game exists and my tiny contribution helped make it happen. It's not like I'm shut out of the content myself, so what's the problem?