Probably because of the way the program was initially organized, and also, frankly, it's an effort to control the workers. They want to know where they are.
That's speculation, but the practice and the result is that employers are able to exercise undue influence on the workers because they can't move from one employer to another, as Canadians can, if they're mistreated. The employer holds a very heavy hammer over their heads, and this lack of mobility is one of the big reasons why these workers are much more vulnerable in the workplace than Canadian workers.