The kind of arguments you raise here have always, to me, basically looked like:
"I want something for nothing"
Or in more detail:
"I want to take something you created and agreed to share with me (on the proviso I like-wise share any changes as an exchange of value). I deem your software valuable to me but I won't be paying you money for it. I'll add something else to it so it fits my use-case. But, I refuse/object to sharing my changes because they're valuable."
It's not just sharing the changes. You have to be really careful how you integrate/modify the source. If you aren't you could accidentally expose proprietary information within those modifications. It's that kind of headache that means it is just easier to avoid.
"I want something for nothing"
Or in more detail:
"I want to take something you created and agreed to share with me (on the proviso I like-wise share any changes as an exchange of value). I deem your software valuable to me but I won't be paying you money for it. I'll add something else to it so it fits my use-case. But, I refuse/object to sharing my changes because they're valuable."