I think that Feedback should be an automatic positive for a seller after one month if the buyer hasn't bothered to leave any yet.

That's ridiculous.. Feedback is not mandatory, buyers don't have to leave feedback if they don't want to.

I agree with you!.... Buyers can not pay and waste everyones time over and over again, and no-one can warn other sellers about this,

Yes you can. If you open Non Paying Bidder disputes when buyers don't pay, the buyer will get a strike. If sellers have blocks set in place to block these buyers, that is effectively warning a seller. If you leave a Pos with a Neg comment, then the buyer can have that removed so your feedback hasn't warned anyone

What good would a seller being able to leave a Neg do? Let's say that a seller only lists BIN items, how would you suggest that a seller look at a buyers feedback before said buyer has even bought from them? If you look at their feedback after they have bought from you and refuse to honor the sale, you leave yourself open to a Non Performing seller dispute .

Another note.. Buyer feedback is superfluous.. All it does is show you how much you have bought.. 

Yes it is nice to see feedback from sellers, but it doesn't mean anything...