rgnrk: It's relevant as it measures the moral ground of the company. Employees are often required to make sacrifices when a company is short on money. This is not the case here; so apparently they do it on principle.
ZFR: Would they have agreed if the game turned out a flop and they were paid only half their salaries because of that? Or would have they demanded to be paid the full amount owned.
Well, this actually can happen legally, at least in my country. If a company doesn't have any money to pay their employees, they simply won't. There's a certain garanteed amount that companies have to set apart for such cases, but it will only cover a small amount of the employees wages. And employees has to work several months with no pay before they can't refuse to work at all.
If it's only a flop but the company doesn't loose money or still have enough money, it would amount to a private contract update if the employees agree to lower their salaries. Though there are laws that make it so the companies can lower the employees salaries with the employee input. The only option is to agree to a fair dismissal with a 20 days per worked year compensation (up to a maximum of 9, I think).
ZFR: You either agree on a salary bonus based on sales and run the risk that the game would sell poorly, or you agree on a fixed salary. Period. You don't make demands later because in hind sight your judgment was poor. This goes for both employees and employers who require their workers "to make sacrifices".
They don't have legal basis for a salary upgrade. Which I guess is why their not at that company and disgruntled. So, no, you don't make legal demands. But you have all the reasons in the world to be disgruntled and complain. Which is what they're doing. That happens when workers make a good and succesful product and the company, not only where paying peanuts before, but don't want to raise their salaries even though those workers where the ones providing them with the money in the first place.
But then, this seems to be a company owned by to people that don't care about games at all (according to the link).
And then, there's the artistic merit of videogames. Which is similar to other artistic medium like movies, for instance. A movie is attributed to the movie director, even though the movie belongs to the company who published it. If you pay minimum salary wages to the director of a movie that makes a blockbuster for you and then you don't want to raise his ridiculous salary for subsequent movies; you have the legal right to do so, but it's also normal that he'd be kind of pissed (and most people would consider that kind of shitty).