This new feature was introduced to stop the many non payments after offers were accepted. So, good in that way, but it's come with many flaws, and not offering a combined postage is one of them.
Some sellers do not even know they've been opted into this, and only they the seller, can if they wish, opt out.
The fee on postage is of course legal, it's been around for years, and if illegal would have been removed years ago.
It's not a scam by eBay, it was introduced because some sellers were using a loophole and not paying any sellers fees. For example, selling an item for 99p and adding a £50 postage charge. The seller received £50.99 and paid not a penny in fees.
This fee avoidance couldn't continue, and now cannot happen , it's a level playing field for all. The few spoilt it for th many.