So...let's actually set up a pretend scenario here. Pretend. We are pretend red teaming here; any resemblance to actual terrorist plots living or dead is purely coincidental.

Let's pretend our terrorist cell is going to spit up, travel to 10 places around the United States, and we're going to do a coordinated strike on 10 government buildings. Probably the smartest thing to do is just...do it at a planned time and not communicate after we split up. But for some convoluted Ocean's Umpteen reason we need to communicate and coordinate. I see 3 possible scenarios here:

1. Leader just needs to say GO to the rest of the team, expecting no reply. So one, very brief, one-way communique.
2. Leader needs to send several detailed instructions over a long period of time, expecting no reply. Repeated, large, one-way communiques.
3. The team is going to gather some intelligence and report back, and based on all their observations the leader will say go. Full on two-way communication.

In all three cases, the internet is the better tool for this.

You are correct in that it is difficult or impossible to remotely detect radio receivers, no matter what the BBC tells you. There's no machinery making a log of who accesses what over analog radio. But the realities of radio equipment and propagation are going to eat into that advantage somewhat.

If we're talking truly coast-to-coast, you're going to need HF. MF/longwave won't reach far enough, you need skywave propagation, and you get that on HF...mostly at night mostly during favorable sunspot activity.

I bet you're imagining most of the team using one of those handheld commodity shortwave receivers that does AM/FM and shortwave, about the size of a pencil case with one of those telescoping whip antennas. That might do for 1 and 2, people hear hams on those sometimes.

The bosses transmitter would need to be a reasonably serious bit of kit. At the very least something like an Icom 706 mobile HF rig plus power supply and at least a two element yagi for 20 or 40m. This is an antenna that's 30 to 60 feet wide. Hams do routinely make do with less, but when you're talking to someone with those crappy little antennas, probably inside a building, I'd want to focus my beam at least a bit. A wire in a tree ain't gonna do.

Oh, and, let's say Boss is in Washington DC. It's possible he can make himself heard in Los Angeles but not Wichita, because the "optics" of the ionosphere doesn't bounce his signal down to the ground in the middle of the continent.

One communique of "Baker this is Oven: Preheat complete, insert the bread. Repeat: Insert the bread." might not be noticed. Or some ham somewhere will hear it and go "What the hell, who's horsing around?" If you don't transmit again, you're probably not going to be direction found. But that big radio tower you've got is a weird thing to have.

If you need to make routine transmissions, well now you're going to have to try some steganography crap. They did recently relax the baud restrictions on HF, but you're still talking about 2.8kHz of analog bandwidth that MIGHT get through. It's gonna look really weird if you're repeatedly sending digital pictures to...no one in particular on a regular basis. Now, to blend in, you'll need some genuine callsigns, because the FCC amateur radio license database is a matter of public record. You use a bogus callsign and you'll be found out. If you're transmitting a lot, people will find you, possibly out of curiosity.

Especially if you're talking about everyone in the terrorist cell communicating, well now EVERYONE has to have an amateur radio license from the government, and fairly large, fairly conspicuous radio hardware. There have been spies caught with shortwave radio equipment, and said equipment was used as evidence against them. Entering the US with a smart phone and laptop is utterly normal, entering the US with a shortwave radio is weird.

OR

Get accounts on Reddit, and post cat memes. Compared to sitting around listening to static on an HF set, that looks way more normal these days. Yes, there probably is a log of what IP addresses sent and received what, but it's really easy to make two-way secret communications look like perfectly legitimate traffic. The equipment required doesn't draw as much attention. Keep the steganography subtle or a matter of "which picture I post" and not doctor them at all, well now it's 100% indistinguishable from people having casual fun. Some guy posts a picture of an orange cat, it gets 30,000 views 975 likes and 75 comments, and ten IRS buildings explode. Do you think the authorities make the connection to the cat meme in the first place?