> If the employees are turning on their personal hotspots and using that, you don't have a security problem.
If they connect anything that lives inside your network, at any time, or that even has a VPN connection your internal networks at any time, you have a security problem. It may be one you choose to accept as a matter of policy, but the risk is very real. Worse. Most admins simply do not have the tools are buy-in to review and monitor systems for gateways, remote console access, or network tunnels that may expose your internal network through precisely such a hotspot or modem access.
I agree that by current regulation you may not run a hotspot jammer. The FCC regulations are quite clear about this, partly because they block other cellular communications and services such as telephones and GPS. But I'm afraid I disagreee vehemently with you that their use does not constitute "a security problem".