Yeah, it's explained here:
"Companies install free solar panels because of a government scheme called the Feed-in Tariff , which means that they earn money for every unit of electricity the solar panels they install on your roof generate. The company earns money from the free solar panels on your roof and you get free electricity - it's a win-win situation. "
I guess it may be worth weighing up the cost of doing it yourself and also collecting the pennies while the sun is shining VS it being "free" but passing up on the cash?
Personally I always think it's better to be in control and a little out of pocket in the short term.
I imagine the catch will be, as you expect, that they won't maintain them for you and you'll end up with a fairly big bill if you need replacements; at this point is when I see the Watchdog situation coming in, as the company may attempt to claim the feed-in tariff cash if the panels are replaced.