You make a perfect point - something that most people on this forum have no concept about.
In general terms : They think that if that hamburger place can only sustain $16 an hour too bad - the award says it has to pay $21 and $30 on a Sunday. So the hamburger place either operates one staff member short and/or doesn't open on Sundays.
Or the price of hamburgers goes up.
The fact that somebody might only want to work on Sunday and is happy to work for $16 or $18 that's also too bad - the shop has to pay the award penalty rate.