With software/games, you are never buying the software/game. You are always buying the license to use the software/game. The difference in buying the CD in a store and downloading LFS (or equivelent) online and purchasing the license is that you don't get to try out a demo with the CD from a store version before paying for the license.
You can't but LFS in stores afaik. Once you pay for your S2 license (24 pounds) they'll send you an unlock code for the demo and you can get going straight away :up: