As of 30 September, 2017 attendance on the website for operations is mandatory, if you do not mark that you are attending you will not be allowed on the server. It takes no longer than a minute to click 'Yes', 'No', or 'Maybe' to let both command and the campaign makers know that you will be attending.
This is mostly due to the fact that a continuing campaign that constantly progresses and changes requires a reliable head count to adjust the difficulty and scale of the mission. No excuses will be accepted barring extreme circumstances. Mission makers and command dedicate hours and even money to keep the servers up and running, please extend a bit of courtesy to at least let them know if you're in or out for operations. If something changes or happens, it is up to YOU to update command.