If you buy a car, and it breaks, you are free to take it to any car mechanic to have it fixed -- you are not forced to go back the one mechanic who sold you the car.

I believe software should work the same way -- so if you purchase software from me, I think you should have the freedom to hire any interested software developer to make changes to that software, and not be locked in to hiring me again to make those changes.

proprietary software is an easy way for software companies to create a vendor lock-in situation, which I think is abusive.