Most tag implications work 99% of the time, but 1% of the time, they are not appropriate.
Example (anthro_on_feral -> bestiality):
https://e621.net/post/show/943120/absurd_res-anthro-anthro_on_feral-anus-armpits-aur
Edit: linked to correct post
The above contains anthro_on_feral, but as there's no actual sex occuring, it wouldn't necessarily be classed as bestiality.
I'm sure there's countless other examples of cases where the implied tag is not appropriate for the content, for this reason, being able to make some exceptions would likely be helpful.
Updated by savageorange