There are several reasons why sellers cannot leave negs, many of them the same reasons that when sellers could leave negs many of us didn't.

It only warns sellers who go in and check the feedback of every bidder, that still doesn't stop a buyer using BIN or a last minute snipe.

A buyer used to invariably leave a retaliatory neg, while they do not impact on a buying account too many on a selling account and it can lessen the chances of getting buyers.

Unscrupulous sellers would leave retaliatory negs if a buyer left one then blackmail buyers who thought negs were important into mutually removing them allowing them to maintain an artificially high feedback score.

A lot of sellers didn't take the time to think what they were typing and left the sort of comments that would have a prospective buyer hitting the back button, how feedback is left is just as important as the type of feedback.

There are other reasons too, it is far better to give non payers strikes so neither side can leave feedback but you can block them and allow other sellers to do the same if they have done it more than once.

As a long time seller I much prefer the system as it is now.