It's very analogous to regulating certain common chemicals as "controlled precursors." It's not that the government wants people not to have these substances, or that they want to restrict their use in making useful, non-controlled substances; rather, it's that they want to know who has these chemicals, so as to be able to attribute and trace any controlled substances that might be produced.

In this case, they want to have a list of everyone who's playing around with radar systems, because any such person could — entirely just by taking the commonly-available tech and advancing their own private understanding from there — become a fully-fledged radar engineer, and build a system that could be interesting to terrorists et al.