Why do they charge fees on postage?  

Simple.  If they didn't, people would sell $20 items for $1 plus $19 for postage, and thus avoid fees.

Poor eBay, you think they're so dumb they can't work out a system to track reasonable postage costs? 

Is it fair to sellers?

Yes. 100%. Ebay fees are a cost of business and should be factored into your business plan and pricing structure.  Business 101

Ok, so you're saying if postage costs me $10, i need to charge the extra % on postage to cover eBay fees? That's good for Ebay but not for the buyer.