Because people are far less likely to see closed source.
Which in turn leads them to thinking long
and hard about the usage of it.
Eons ago, if I wanted to see the code that produced
an internal file for an application so I could write
my own interface to it, I needed to call the vendor.
The vendor would FedEx me some very serious looking
legal documents about usage restrictions. I would
pass them on to the legal department of the company
I worked for. The lawyer would quiz me, make sure
I REALLY needed to see it, and would make sure I
understood the legal problems that would ensue if
I did anything that would allow the vendor's source
code to be passed on to anyone else. I would think
for a bit, and agree I really did need to see it, then
the lawyer would sign it, my boss would sign it,
and I would sign it. Everyone kept copies.
We would then FedEx the doc back to the vendor, and
wait. And wait. And wait.
The disks would show up, I would copy them to my system,
and my boss would remind me to be careful.
I would review the code, pull the file structure out
of it, write a bit of code that used the structure,
test it, and then destroy my copy of the vendor's code.
I would then notify my boss and the lawyer. The lawyer
would send a nice little letter to the company stating that
the code had been destroyed.
The whole process would take months, multiple people were
involved, and an internal enforcement procedure (me getting my
ass canned) would be triggered if there was a hint of that
code passing from me to anyone else.
