Here is a fuller explanation. You are thinking like a bookie not a BF layer.

If I put 16.50 on a 2/1 shot I expect to win $33. What could be simpler than that? I get my $16.50 back and the bookie pays me $33 of his money. You have misread my post to think that I don't expect my stake back if betting with a bookie. Of course I do. Where you are confused is that when laying on BF the backer does not actually give me his money up front. When the race is over I either take his 16.50 or he takes my $33.
The reason that the bookie pays out $49.50 is because he has taken the backers money up front. That does not happen on the exchange. BF hold both amounts until the contest is decided. I'm sure you haven't had 29 years laying on BF.