well, they don't charge you for using their infrastructure to lug your juice to them. (or they charge you by paying less.. effectively)
do you have some whacking big energy storage that allows you to store and use the energy at night that you generated during the day? (ie.. they effectively charge you for acting as your battery)
i think in the uk, selling solar back to elec companies is basically gov subsidised (and the subsidy was slashed a while back when lots of people signed up for it or some such.. i think.)
the way it works (mostly) is that they (the installer? companies / energy companies?) install their equipment on your roof. it's not yours.. and there's some sort of contract for X years or some such.