That's fair for software that's a one time purchase. But most modern software is subscription based, and is more akin to renting an apartment where you're more restricted in what you can modify.

Legally, I agree that we should be able to modify whatever software we purchase so long as we don't redistribute it. I don't believe, however, that the engineers should be required to hand over the source code for us to do it.

Car mods are usually legal, but no one is demanding schematics from the manufacturer to help them go about it.