"But doesn't every scientist have to be subjective sometimes? Suppose there are two competing theories concerning a given observation."
If there isn't enough information to make an objective choice between theories, then the issue is undecided by science. Scientists who do take sides are therefore doing so for extra-scientific reasons.
"Aren't Hawking's frequent atheist rants subjective?"
Yes.
"No one exorcises him from the science community for delivering these rants, even though he often delivers them as part of an otherwise scientific commentary."
Because his science statements are not of this nature.
But surely scientists take sides in debates over competing theories. Why would that be wrong? Merely to do research on one theory and not the other would be a subjective decision. Humans can't help being subjective to an extent.
The thing is, a scientist could deliver a purely objective discourse on nature, and then conclude it by subjectively stating that his observations have led him to conclude that nature is all that exists and there is no God, and it wouldn't raise many eyebrows. But a scientist who followed a similar objective discourse with a statement of faith ("My observations of the workings of nature have strengthened my belief in a deity") would likely become marginalized in the scientific community.