They only take it from the seller because as a buyer you'd be annoyed at paying a 3-4% extra tax on every transaction... maybe not a lot for small amounts but imagine paying ÂŁ1000 for something then miraculously it's ÂŁ1040 when you pay.
This would result in buyers thinking twice, and they don't want that as they'd be missing out on their 3-4%, so they take it from the seller instead as it's easier to take from something already transacted.
It's annoying, but what's more annoying is eBay invoice you their fee's on the full amount transacted from the sale, not taking into account that paypal take 3-4% too, over a barrel it seems.
If the buyer wants 'cover' or buyer protection, pay the money, it's like buying a car and expecting the insurance included.