I have not seen anything to suggest the club will FORCE STH's to purchase cup tickets you seem to be getting stressed about something that has never been suggested.
Listen up Gazbag, why don't you let people have an opinion. It may not be suggested now, however the club will always look to sell as many seats as possible with the least amount of cost. By selling as many STs as we do,makes it a lot easier to sell match day tickets as a bulk of the seats have already been pre-sold. The next step would be to ensure those STHs buy their own seats for cup games, so that's why they have introduced the membership schemes. They help the club to pre sale cup and away games. Its in the clubs interests to have all STHs pre buying cup and if needed playoff tickets, it means that they start with at least 23, 000 tickets sold before any draw for a cup game. I am not saying it's going to happen, however I do believe Man U do something similar with their STHs for cup games.