You have to read between some tiny lines here, to see what you are buying, and this is 100% legal.
The actual 'sim part of this package' is free, and fully supported by the people who make FlightGear. As long as The people behind FlightSimPro send any and all source code changes back to FlightGear, they don't care what you do with it. You could be doing this yourself.
What you are paying for with Flight Sim Pro is:
-The DVD's, their time, and bandwidth.
The amount of time you don't have to sit downloading all of it from the internet. This is WAY more valuable than you may think. Some countries don't have the benefit of unlimited download bandwidth, and simply cannot download the free version. The world scenery for FlightGear takes about a month to download if you want it all. They also have included all of the planes and other addon bits that are currently available.
"But wait, they have a download version, why would I pay to download, when I can do it for free from FlightGear?"
A: Bandwidth, and content. What you get from FG is the bare bones, San Fransisco area only, with just a couple of aircraft, from a very limited bandwidth portal. Once again, you are paying for someone else's time here. They have put in all the work to get you FlightGear at it's best possible state, saving you a TON of time and bandwidth usage (again, VERY important in a lot of countries around the world.)