Cricket is licensed under the GNU General Public License. This means that it is a copyrighted piece of software with specific rules governing its distribution. In particular, these rules guarantee any user of Cricket the right to see and modify the source code. There is a lot more information about copyleft available from the folks at the FSF.
Briefly, here's my understanding of the implications of copyleft:
That last one is important, and bears more discussion. Many commercial organizations are using Cricket or systems derived from it and RRD Tool to provide valuable quality of service data to their customers. This is great! I'm proud to have helped make it possible for people to share this kind of data. However, I ask that people who are making money using Cricket consider returning some of the value that you get from Cricket to the community.
You can give back by being an active member of the Cricket community (submitting code, helping with documentation, or answering questions on the mailing lists). Or, you could make donations to a charity on behalf of Cricket users everywhere.
I reserve the right to publically expose you if you are in violation of the GPL. Since he's more eloquent, I'll let Philip Greenspun explain my philosophy regarding this type of situation.
Jeff R. Allen