B&H offers a variety of shipping options. The one I always use is UPS ground, and in all honesty, I haven't paid a lot of attention to the other options. (I'm in Rhode Island, and UPS ground from B&H usually arrives in a day or two, so I don't care to pay three times as much for overnight shipping that'll likely arrive at my door the same time as ground would.)
My understanding is that B&H is just being very conservative about this; many types of products go in their "will not ship" category until they can check out their contents in detail to be 100% sure that they aren't restricted. Moving items out of this category doesn't seem to be a big priority for them. They also don't want the hassle/risk of setting up a system to ship hazmat items via some methods but not others.
Other retailers are happy to ship most of these items. Adorama and Freestyle, for instance, both ship most of the things B&H won't ship, although they do restrict it to ground shipment (UPS with Adorama, FedEx with Freestyle). I've never noticed a specific hazmat charge when buying such items from either place, although it might have been bundled into the "shipping" line on the invoice without further elaboration. AFAIK, they aren't breaking any laws; it's just that B&H is keeping very far from breaking any laws.