Just a quick google search turned up this:
Injury Liability for Slip and Fall Accidents on Icy Surfaces - AllLaw.com
My non-legal option is they made the offer to not have to waste their time, if you lawyer up or ask for more, they'll make you take them to court.
It all depends on your local laws and weather, you ever try and keep a large parking lot in a cold weather state clean as a Florida summer? Just not going to happen, that's why lawsuits for icy falls don't occur very often.