My guess is that the are requiring this in order to reduce the amount of fraud there (I am sure there still is some, but...). Apple really does not want to be involved when someone can't get into the Taylor Swift concert that they paid some scammer a lot of money for the Apple Wallet ticket they got.
Having an authenticated developer account at least provides some level of speed bump to scammers, and a better starting point for the police.
There are many events that are still sending you a pdf file with your tickets. Until fairly recently, that included major venues too.
The charitable explanation is that the wallet is designed for credit cards, and tickets were an after thought. Though I suspect it is really Apple trying to keep a walled garden, just like they always have.