Why doesn't the castle owner just kill any trader who doesn't give him
a cut? That's the way it worked in history.
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In real life, your nobles could operate on fuzzy commands: "Kill any noble
who doesn't give us a cut." In this game, you'd be a turn too late.
Automating that would give you that sort of RL similarity, but you'd also
have to allow the trader to refuse to pay and be attacked (and set the pay
tax 0 flag, to pay _any_ set tax).
As much as my eyes glaze over when I read the laundry list of threats
posted by landowners in the Times, I think that it's better to handle tax
that way, person to person, than put it in the game engine.
I think the question is not "Would it be fairer to / better for the castle
owners to do this?" " but rather "Would it make the game more interesting
/ fun?" Just adding a tax would not improve the game, and there is a way
to do it on the players' own initiatives.