Z:gnu-www-ja-gpl-faq--c95c02-There are two reasons for this/en

There are two reasons for this. First, a general one. If we permitted company A to make a proprietary file, and company B to distribute GPL-covered software linked with that file, the effect would be to make a hole in the GPL big enough to drive a truck through. This would be carte blanche for withholding the source code for all sorts of modifications and extensions to GPL-covered software.