( I mostly see coleslaw being given when ordering burger and fries so I'm gonna use fries as a example but I am aware coleslaw may be given out with other dishes.)
Ever order a burger and fries and just notice how almost every take out place, mostly local ones, automatically gives out coleslaw? I know some people who eat coleslaw and therefore enjoy getting the dish but the majority of people I know don't like it or prefer if restaurants didn't automatically give them away. You could just request to have no coleslaw when you order the food but you never do as you don't ever think of coleslaw enough to do so. Now, most people like cream cheese but you don't see Dunkin' Donuts or Tim Hortons spread cream cheese when you order a bagel unless requested. So, why is coleslaw given out like confetti? Well, I don't believe takeout places give out coleslaw for the kindness out of their own heart for the small minority out their but rather as a way to give out less food.
Think about it, everytime you get that plastic container filled with coleslaw, it kinda just leaves a empty space in the take-out container. While this space is pretty small, that could have been filled with more fries, giving you more value for your money. Coleslaw contains ingredients like cabbage and onions, both being common in many dishes these restaurants serve, making it so you can make a bunch of coleslaw using scrapped onions and cabbage for free.
Now, sides like fries, while still simple and cheap, still cost money to cook. By adding that dreaded plastic container of coleslaw in the take-out container, you have to give out less fries. If you ever heard of the American Airlines olive story, you can likely understand where im coming from as even just a few less fries can save businesses more money in the long run. Not only does this save money for the restaurant, it also has the potential to make more money as the customer who threw out his coleslaw won't feel as full now, making them a repeat customer.
While I am sure this tradition was started with good intent and most people haven't even thought about this, I'm sure giving out coleslaw so restaurants can make less fries, whether knowingly or not, saved these places lots of money in the long run.