Really! But what's the legal argument there? Nothing is copied, it's a mod.

Copyright includes the rights for controlling the modification of the work.

Questionable. It certainly prohibits people from selling modified versions of the work, but it's doubtful whether that prohibition can extend to you choosing to modify it yourself.

In what sense, though? Publishing a modified copy, sure. But why extend that to a thing-that-modifies-a-copy-you-have?

Markers are illegal because I can use them to cross out words in books I've bought?

I don't think there is any jurisdiction where markers are illegal.

is that really what OP's argument was about?