Is the use of a comma appropriate here, and can there be a clause after a comma without a subject?

I am sure that this is very simple, but apparently it is not for me. Hopefully someone can help.

i am proofing a book in CMOS. Often the writer extends a sentence by using a comma before a second clause without a subject:

The courts want to make sure that justice is done, and sometimes are disappointed by the slow pace of change. Is this correct? If so, why?

Thanks very much.