That's not an ever-applicable rule. He was like 10 feet off the base, it would have been an easy tag out for Gordon. Pitchers should never be that involved in run downs.
The risk of Bills throwing it back to SS from the mound are much higher than him simply running at him. And it would not have been an easy tag at all, Weeks having 10 feet off the base and then the time it takes for Bills to throw it to 2B, Weeks has a signifcant lead on Dee and can easily make it to third before him. Every single coach would have Bills do exactly what he did, in fact he should have just tried to tag him out, his fault was throwing it to Dee at all