How do they get away with this?

Yes it was pretty frustrating for me as well. What you have to do is when you hire them, lock on to the targets using LT, make sure you have your hidden gun selected then you have to tap Y, then it will give you the option to lureaway, or attack. for button
B I find that the lock on target doesn't work as smooth in this game.

do u know how to do it on pc?

This has been hard to get right. I have found that if I hire a group, get them close to who I want them to distract and make them wait. The, when you get really close to the guards you can use the LT like before and one guard will get the red bar above their
head. Then the B button should work. This has been much harder than AS2.

1. Have all the thieves NEAR YOU!

2. LT to target someone to lure.

3. Press B as it will show Lure instead of Stop if all thieves are within eyesight of you.

Finally got it to work after about an hour; if you double tap the LT and then hit B it will work. It may take some practice