As I have said before the single line thermostat is for controlling the heat only.
The requirement for a disconnect is another requirement all together. There are multiple ways to provide a disconnect for the baseboard heat. It is your choice as the installer to make a decision on how you would like to acheive that requirement. The choices are listed in table E4101.5.
The fact that a homeowner comes in later and buys the wrong thermostat or removes the lockout that has been provide in the panel are different problems and issues. the single pole line voltage thermostat is not allowed to be a disconnecting means for a 240 volt circuit but is a legal piece of equipment even for 240 volt systems.