Work Permit And Residency

I have a work permit based on my non-immigrant Category B visa. The duration of my work permit matches the duration of my visa. So, for example, if my visa was cancelled, my work permit would automatically lapse as well. My understanding, therefore, is that you cannot have a work permit without the underlying visa.

Now, my question relates to premanent resident status. I understand that when I obtain PR I no longer need to to have a visa. I also understand that PR allows you to remain in Thailand indefinitely (unless, of course, it is revoked). So, does the above analysis apply - ie the work permit follows the life of the underlying document which allows you to remain in Thailand? If that was the case, the work permit would run for the duration of your PR - ie indefinitely. But I understand that even if you have PR you still need to renew the work permit on a yearly basis. Is this correct? Does anyone know why this is the case?

Also, I was wondering if having PR makes it easier to renew your work permit?

Thanks,

Bobcat

PS Sorry about all these questions about PR, but I am very interested in how it all works.