Commas only mimic the pauses that occur in natural speech. There are few (if any) real grammatical rules that govern their use. You can create a sentence entirely without commas if you like, or add commas in unlikely places if you want to upset the reader. It's entirely a question of style.
The comma codifies a brief pause, while the semicolon and period indicate longer pauses (and possible changes in subject/tone). Again, while there are style guides that suggest certain patterns (to make the sentence more readable), it's fine to ignore these and punctuate the sentence as you see fit.
This example demonstrates an oratorical style that, by pausing before and after the "therefore", adds some measure of gravitas to the second part of the statement, imbuing it with extra significance. However the writer uses the conjunction "and" to link the first and second parts of the sentence -- possibly to imply the second is a natural consequence of the first, and beyond argument.