The thing you're missing is that once they take ownership of the case (Because of the Jewish centers AND its interstate/multistate nature, both of which are reasons they would assert jurisdiction), everything else concerning the same dude for which there is a Federal offense will get thrown in the same pot. Just because they might not normally go after the non-religion-connected threats by themselves, does not mean they will ignore them once they open the case on the religion-linked ones. Personally, I don't think they'd necessarily be uninterested in the non-religious-connected threats, if there was a large number of them over multiple states, but the FBI field offices and US Attorneys (As well as the DC HQ of both) have a very large amount of latitude over what they choose to pursue, and they exercise it kind of unpredictably at times.