When there are minor updates to a program, i.e. 3.xx to 3.yy, a subscriber is not charged for an update, but when a major/large update to a program, i.e. going from 3.xx to 4.0 occurs, there is a charge as the program is significantly different in its features, and that continued development is based on time and labor by the devs. Clearly, they don't work for free, and neither does anyone else. 

There were offers on discounts for prior Fences owners, as well. There still are:

https://www.stardock.com/products/fences/

Also, your Fences3 continues to work, no?