That is an interesting point, and you are probably not alone in that opinion. From a logical point of view, it makes no sense to me, though. Just view it as a purchase that costs $X, but where the author of the software provided you with a voucher worth $X. Why should not paying anything for the software give you the right to modify and fork it as you like, whereas you accept that constraint for software you paid for? Just accept that there is free software which is not open-source. You don't have to "buy" it.

I think my thought process goes: I prefer free software (as in freedom, not beer). But, sometimes the author wants to charge money for it so they restrict that freedom to protect their business. I have yet not fully grasped the author doesn't want to charge money for the software but they restrict that freedom anyway.