That can happen with any ticket except the ones that are transferred directly to you. If you have trouble, stubhub will charger the seller from 40% of sale up to any amount they decide suitable. Therefore, it’s in the seller’s best interest not to mess the sale up. That’s the risk you take when buying from secondary marketplace. Sellers just want to get paid. They are not usually interested in seeing Taylor Swift. The fan protect guarantee covers you, but you already know that.
Honestly, it's because transfers are a pain in the you know what. They are certainly safer but many Taylor Swift shows didn't allow ticket transfers or if they did, Ticketmaster decided to stop allowing them. I think all shows should have ticket transfers 100% but you'd be surprised how many events don't allow transfers. That makes mobile QR codes much easier to sell. Also, in my experiences, many buyers are slow to accept ticket transfers and I have to follow up with StubHub or other marketplaces multiple times about the buyer still not accepting the transfer. I don't have time to follow up on these all the time.
Personally, for me it is a) unethical to do this and b) I sell tickets for a living and I wouldn't be in business still if I was dishonest and sold tickets that I ended up using for personal use. Obviously, this isn't the case for all people.
I sell tickets too (not for a living, but some to make extra cash for my daughter's college). I started using the screenshots recently because to be honest I'm worried Ticketmaster will decided to change the delivery method. If we sell a ticket and then have to change the delivery method due to TM, it can cost us money if the buyer doesn't accept it. I wouldn't be worried about the seller trying to use the ticket. Remember in order to sell we have to follow rules and if we break the rules, we can be charged ALOT of money.