I think that some sponsors see affaliates as "just" a traffic source. Affiliates are actually your salesguys and they put in a lot of time and effort to sell your product. I think the 50% is very fair. Affiliates have to invest in domains, software, writing salestext, etc...

The smaller affiliates, who are not sending one sale in a week, might be selling 10 memberships to your competitor. You can't judge an affiliate on your own stats, as most affiliates have dozens of sponsors. Looking at my own situation; I make several sales a day with some sponsors, while there are sponsors I only make one sale a month with. Of course I send more traffic to the first category.

The affiliates that only put up a banner actually gets less rewarded than the affiliates that think about there SEO and salestexts. The banner proberbly converts 1:3000, while a good salestext can convert 1:200. In that way an affiliate gets paid by the effort and time he is putting in. To become a succesful affiliate you have to work hard, be smarter than your competition and spend a lot of time.

In the end, I know the affiliate needs the sponsor to be succesful, but the sponsor needs the affiliate as well.