Why is this use of "therefore" considered a restrictive adverb?

So, I found this explanation of when and when not to set off adverbs with commas.

Here is an example in which 'therefore' serves as more of an aside or a pause:

All of the test animals, therefore, were re-examined.

In this case, 'therefore' is bounded by commas to separate it from the rest of the sentence and to provide a pause for the reader. In this example, if 'therefore' were moved and placed within the verb 'were re-examined', it would be treated as an essential (restrictive) adverb and would not require commas:

All of the test animals were therefore re-examined.

But, I'm confused as to why 'therefore' suddenly becomes restrictive when inserted into the verb, 'were re-examined'. It's still the same sentence in meaning, right?

My guess as to the reason why was that we don't want to use a comma or commas to divide the words that comprise a compound verb. But, then I thought of an example where it would be fine to divide the words of a compound verb! Here it is:

If Star Wars were, for example, deemed inappropriate for children, Disney would make relatively less money.

So, my question is why does 'therefore' suddenly become restrictive when placed between 'were' and 're-examined"?

I don't think using commas around therefore has anything to do with the adverb being restrictive or not. Even without the surrounding commas, therefore is not restricting anything. It merely connects between a prior sentence to the current sentence with or without the surrounding commas.

With the commas it is presented separately from the main clause, whereas without them it is presented as being integrated into the main clause.

I agree with JK2 but would add that all adverbs have a "natural" position within a sentence and this position does not require commas:

[Nothing was found,] therefore all of the test animals were re-examined.

[Nothing was found,] all of the test animals, therefore, were re-examined.

[Nothing was found,] all of the test animals were, therefore, re-examined.

The extra-position of adverbs adds emphasis to the adverb. In written English this emphasis is displayed by off-setting commas.