They don't realize it because the rates are the same whether you get a free phone or not. If you buy your phone you'll be paying the same monthly fee as some other person who got his for free.
The plans that subsidize their phones start from $60 a month for unlimited, and the unlimited plans that don't subsidize phones start from $50 a month.
This may, of course, vary from provider to provider. I'm only familiar with T-Mobile.
http://www.t-mobile.com/Shop/Plans/Cell-Phone-Plans-Overview...