"Honestly, who cares if CC permit holders with firearms slow down an investigation after the fact?"

The shooter was still at large, thus it was not "after the fact." As far as police knew, the Walmart shooting was just the first part of a spree in fact, they probably were working under that assumption. So who cares? I'm guessing the whole community in the area who only knew that someone shot up the Walmart and that killer was still on the loose.