They used to offer the exploits free because the devs would rely on donations from the community. Like said above there was a bounty thread for s-off where folks would pledge money to help the devs. Problem was historically once the exploit was found only about 25% of folks would actually fulfill the pledge. This would leave the devs to eat the cost of bricked devices and the time and frustration included. Lastly there were a lot of folks who would complain, demand and try to belittle the devs for taking so long etc, etc.
Firewater was released in the spirit of the android community, and looking through the threads for Firewater and the bounty thread, there were a whole lot of ungrateful people demanding an update for the exploit. It was a slap in the face for the devs. Also this device is more complicated which resulted in a lot more bricked devices. They shouldn't have to front their own money, so they had to find a way to get their investment back.
Remember, if Sunshine can't work for your device, they won't charge you.