Why do you think it is illegal?
'Forcing' an individual to work for no pay may certainly be illegal (or one would hope - I'm not 100% on the legal definition). However there is a big difference between this and agreeing to work for nothing or not walking away when you find out 'you are one of the privileged few doing it for love'....... It's a free country, to use a cliché, and one can't be forced to remain in slave labour for little or no pay - it's a choice! (just talking aviation here so don't get all low socio-economic classes with no options etc etc on me).