Do you not see how complicated it would be to implement? WP has no login system, so how do would you confirm what coins you want to not trade?
It could be coded in the worker username or password (/DOGE={address} or the like). Not sure if there are length limits on those though.
I kind of disagree that buying the coins yourself is a direct substitute. At that point trading costs are being paid twice, which is dumb.
Development priorities is certainly a valid reason to implement one feature over another though.