That's true: if you choose a spa without a subpanel you will have to install some sort of cutoff switch. That's one of the many reasons that HotSpring includes a sub panel with all 220 model spas. It protects the spa, conduit and users, splits the spa into two separate circuits (so the pumps can continue if the heater has a problem), helps with troubleshooting, and fills the requirement of a local cutoff switch.
The panels also have room for another circuit breaker so you can add a courtesy outlet, which is also required by code.