Does the iPhone's passcode protect the SIM?
If you did not use a dedicated SIM-Pin then your SIM is not protected.

In other words, if you lose your iPhone someone can easily take the SIM card out of the iPhone and use it it in a different phone.

The iPhone PIN only protects the contents of the iPhone NOT the SIM card.
You have to use a SIM pin to protect the SIM card, it's available in the iPhone settings.

So if you lost an unlocked/unprotected SIM card, you should immediately call your provider and cancel the SIM card. They can send you a new one.