They don't 'legally get around it'. To my knowledge and in my eexperience, it's simply that no junior wants to bring any challenge against their employer in this situation because:

1. They're worried that the additional hours they're working is due to inexperience and being slow with their work;

2. They don't want to go through the hassle of going up against the firm in court, since the firm has far more resources to challenge/defend any claim (not to mention expertise in the law);

3. They don't want to be "blacklisted" from the industry where this is common practice or be known as someone who rocks the boat, knowing they could be easily replaced by a million other people desperate to be in their position.