50% seems like a big hit, so for these to make business sense, the merchant must either make up for it by people buying more than the minimum, getting repeat customers, or getting some cash from people that pre-paid and never bothered to claim their goods/services and having that subsidize the others.
Also, speaking as a customer, you should not be bothered by those who use the deals without buying more than the minimum. It irritates me to no end when someone offers a deal to promote their business or to try and hook you, then treats you like a second class citizen for taking advantage of the terms that they offered. If I like it, and feel that it is a good value without the discount, I will come back.
Realize that many of these offerings can push things into different affordability ranges, so you will definitely get non-repeat customers just because the deal is too good to pass up. Granted, this probably doesn't apply to a "$5 for $10" deal.