A linux distribution is not just one product if you look at licenses. You have some GPLed software and some distros might have some packages with other licenses. Once example atm is XFree86 4.4.0 version with their new license that's not compatible with GPL and which is included in slackware. You might have some distribution with commercial parts that you can't use but that still has GPLed parts that you can use.
If you use software in your distribution that's under GPL then it's very smart to read that license.
If you use software in your distro that's under some other license then read it very carefully. The license-stuff is a hassle at times. GPL is really nice unless you want to make the software unfree...
Check here for a faq on GPL http://www.gnu.org/licenses/gpl-faq.html