Not at all!
While it can be advantageous to join the Be developer program (see a previous question for details of program benefits), there are plenty of folks developing for the BeOS who are not in the program.
The primary benefit of joining the program is access to our Developer Technical Support group. But everyone, registered developer or not, has access to the technical and other information we publish on the Web. This includes the Be Book, which covers the BeOS APIs, sample code, various tutorials, the Be Newsletter, etc.
Browse the various resources we make available to all developers, registered or not. There is very little that is restricted to only registered developers! And if you have suggestions for things to add, please tell us!