When you purchase a ticket for any ground, it is subject to the club's terms and conditions of entry. You are entering into a contract whereby you agree to adhere to the rules set out by the club. Whilst inside the stadium, if you are found to be persistently standing, smoking or doing anything else that is against the rules, you run the risk of being ejected from the stadium as per the terms of the contract that you have entered into with the club. If you are seen to throw objects onto the pitch, attempt to enter the stadium whilst clearly intoxicated or are seen consuming alcohol in the stands, you run the risk of being arrested, as these are criminal offences. Racist or homophobic chanting are similar and most clubs operate a zero tolerance policy.
You can ask for a copy of the ground regulations when you purchase your ticket and clubs must also display them at the stadium, so there is no real excuse for ignorance or doubt.
There are certain parts of pretty much every ground where it is generally accepted that people will probably stand up. Lower Gwladys is one of these areas and it's pretty unlucky if you are ejected from there. The club are, however, perfectly within their rights to do so, especially if as the letter would imply, previous warning(s) have been issued. The general messages over the PA system count as a warning too btw.