There is often an assumption that the buyer has fulfilled their obligation when they paid, meaning the transaction is half done (done as far as the seller is concerned) and the seller should leave feedback.
That is true if everything was perfect, but sellers should deal with reality. You have buyers who do chargebacks, complain that a corner of a magazine has a small ding in it (and therefore cannot possibly be new, meaning you are a fraud for selling it as new), or go off on you for terms they agreed to (check my "why I hate ebay buyers" thread to see what I mean). Due to the ability for a buyer to extend the transaction beyond simply purchasing and recieving, any seller with conern for their reputation cannot, without serious risk to themselves, consider the transaction done on the buyers end when they recieve payment.
I do honestly believe that some buyers would have left me negative if I had already left them feedback. For example, one buyer didn't recieve an item after 5 days and (rudely) asked for a tracking number, saying he didn't think I shipped it. I told him I purchased one but lost it (in my listing it says the buyer must add 45 cents if they want it, but I purchase it regardless). The guy then went on a rant about how he worked for ebay, said he would have my status revoked, called my a thief, and filed a chargeback with paypal. I actually found the slip one day before the period I had to give paypal the info was up. I found out that the buyer recieved the package the day after filing the chargeback, and just left it up there in hopes of getting it for free. I am sure with the chargeback and everything else a neg would have been thrown in if he wasn't afraid of retaliation.
I had another guy who it took 2 weeks for him to get a package. He kept insisting I leave him feedback and I kept responding that, due to past incidents where buyers unjustly left negs or buyers getting positives then filing chargebacks, that I no long leave feedback before the buyer. This went back and forth for days, him getting angrier each time. Finally he grudgingly left a positive.