It's a bit like having a job and your employer is ITTF.
In a normal job if you don't go you because you are sick you must present a doctor certificate.
It makes sense that they force them to go, otherwise nobody would go to WTT except the chinese who always win lol.
FIFA is similar....if they call you to play for the national team you must go and your team must let you go and you can't play for your team during those days.
Bad comparison, but it proves the point exactly.
FIFA World Cup is once every 4 years. Continental Cups (UEFA European Championships for example) as well, so that is one international championship every 2 years.
The calendars are the same for every player in the World, so planning is easy.
None of it is mandatory. Your team calls you up. If you refuse, your national association might get angry, but that's it. FIFA is not even involved. They just recieve a entry list of players from the national Association.
The differences:
ITTF already had a yearly World Cup. That is what was in 'the contract'. It wasnt mandatory, just like in football.
ITTF WTT then added 5-20 events per year. Mandatory. Money and ranking penalties if you refuse.
Its more like ITTF is your employer and then said: I know you signed up for one world cup per year, but now there are 20 events per year and you need to travel to all of them. No income for you buddy, only costs. Yes FIFA added a dumb Nations League as well, but players do get paid there.
Do you think people from many developing countries can afford to fly over the world? That is why the racism card is pulled. If you are poor, you will never get a good ranking. That is always the case, but much worse now. The point penalties alone will keep your ranking down. Olympic entries are based on these points I believe (might be wrong), so I guess also say goodbye to ever being an Olympian.
The employer comparison is even worse, because employers pay their workers, like Tony said. It's work for pay. In this construction there is only work, no pay. Instead there's punishment. Either go, lose money on travel and accommodation and missed income, or don't go and lose money on penalties and penalty ranking points. Awesome.