They prevent unauthorized people from going behind lines because they seek to avoid what they call an incident within an incident.
People who are not firefighters are ill advised to try and fight a fire. I work in the field and will do certain things with regard to wildland fire, but I would never attempt to fight a structure fire. There are plenty of places I will not go, structures I will not protect, roads I will not go down because they are dangerous. The average homeowner does not have that knowledge and many who do make poor judgements when it comes to their own property.
On these fires, I’ve seen people injured entering burn sites, falling through weakened bridges, rolling vehicles off roads, falling into compromised septic systems. I heard of someone stuck under a care trying to rescue a cat. And that says nothing of the looters who enter under the guise of helping.
When these people are hurt, services are taken away from firefighting to address their idiocy.
No one can kick you off your property, but once you leave, they can be prevented from reentering,
A simple rule of thumb for a safety zone is 4 times the flame height. Should those 75 foot trees in that area torch, you can easily be talking 150 foot flames. Very few homes I googled in the area of this Texas fire had that. And that 4 times distance assumes no wind and flat terrain , which of course is never the case.
If you have a red card (training) you can get authorization, particularly if you are with an agency or nonprofit. There are many groups that county agencies and sheriff departments authorize to enter. But if you just show up, you will be directed elsewhere to help.
Download the safety zone app.
https://wise.wildfireanalyst.com/
Apart from Olympic level stupidity with regard to the authorities' response to the Lahaina fires, it appears that the post-fire actions by police and other officials in Hawaii and Canada have been more about control than assistance.