It's important to not equate sending SWAT "into a shots fired / hostage / bodies on the ground situation" and a situation where it's been claimed by an anonymous phone call that something like that might be the case.

It's not the same. It's reasonable to respond with lethal force after shots have been fired by the opponent, it's totally not reasonable to do so when you suspect that it might be the case that they might get fired with no good evidence that it actually is so.