I mean, if there's a reason why that's the case I'd like to hear it, but still, I think having actual tags for Oleander, Pom, Shanty, Tianhuo, Arizona, Velvet, and Paprika would probably make it at least somewhat easier for some people to search up fanfics about them.

They aren't from My Little Pony. Why would they?

7575913
Ok, fair point.

7575898
Fimfiction has a rule that all stories must be related to the universe of My Little Pony, which would be a reason not to have character tags for the TFH characters... although it is a bit ambiguous in the case of TFH since the story approvers have allowed pure TFH stories with no pony content at all. My guess is that they probably won't add any non-pony tags. You can still search for the Them's Fightin' Herds tag however.