So basically they're building a second internet / wireless communication network, which only the government has access to?
Well this way in case of unrest I guess they can temporarily shut down the internet and other communication networks completely and yet still be connected to each other.
This isnt that, for 6b dollars, you're unlikely to cover more than a handful of major metros, figure its 250k for a greenfield site, 100k for a co-located one, plus associated core network costs, for a national network you'd be looking at more like 100b+.
Beyond that, most communications between the field and dispatch is narrowband, not wideband, and this is unlikely to change in the near future - if you think the police agencies are relying on public internet access now to do mission critical dispatch, you'd be wrong.
"Taking down the internet" would require some really extreme methods, on the order of EMPing large swathes of the US. It's unlikely that anything less would seriously disrupt government comms for more than a day or so.
If this network is actually going to be resistant to "taking down", they could have just as effectively used this money to upgrade the public internet backbone as needed.
Most agencies already use the internet with VPN's and the cell system. This will give them a dedicated LTE radio band but I think the towers and backhaul to inet are the same with some sort of prioritization.
If you are the skeptical of the government type though it is true they could shut down the public bands and keep band 14 up. Probably not though, they will just just get backhaul priority and the public bands will be jammed up if something bad happens.
They are probably building the base network for $6B.
New York tried (and failed) to build a similar network in the early 2000s... and the cost was supposed to be like $2B. For the whole country... that's easily a $100B project.
There's a lot more to this than just having "another internet". They need to be able to collaborate with multiple jurisdictions, handle overlap between jurisdictions (i.e., State Highway Patrol, County Sheriff, city PD, various federal agencies, from GSA and VA police to the FBI), hook into other radio systems, etc.
There are already systems in place to allow essential communications to take place during crises on the public networks. People get issued an ID number that will give their call priority.
Well this way in case of unrest I guess they can temporarily shut down the internet and other communication networks completely and yet still be connected to each other.