Things I Didn't Know Were Illegal

In an otherwise straightforward story about US born jihadis, the AP has this tidbit:

Several of the defendants, including Boyd, were also charged with practicing military tactics on a private property in Caswell County in June and July of this year.

I wonder what the definition of "military tactics" is? And why would it be illegal on private property? If I drill, march, or practice convoy ops, that might be weird but should not be illegal. If I want to practice shooting, play airsoft or paintball (as I have done many times) is it breaking the law because the tactics might be military?