Z:gnu-www-ja-gpl-2.0-faq--702df3-=3Ca href=3D"=23MoneyGuzzlerInc" nam/en

I'd like to modify GPL-covered programs and link them with the portability libraries from Money Guzzler Inc. I cannot distribute the source code for these libraries, so any user who wanted to change these versions would have to obtain those libraries separately. Why doesn't the GPL permit this?