If you have another teen in your house, the risk is not between your teens, it's the fact that the person coming into your house is exposed to you and your family and vis versa.
Say you have coronavirus but are currently asymptomatic. Your child hasn't currently caught it from you yet, but they bring over another teen. The teen catches it from you but is asymptomatic. They go home and pass it on to their parent. You get symptoms and test positive- the teen might isolate (after potentially 3-4 days of going to school etc) but the parent won't. They go out to work, and infect someone else, and it helps keep the virus spreading.
Equally, say the teen has coronavirus and is asymptomatic. Now, you might expect that your child would catch it from them regardless, but transmission doesn't always work like that. Say your child had an asymptomatic case a month ago and has immunity. But you didn't catch it and their siblings didn't, so when the teen comes into your house you all get infected. Until you start to show symptoms five days later, you don't realise anything is wrong, and you've all being going about your daily lives, infecting more people.
By not having them in the house, you increase the degrees of separation between you and them, and transmission is slowed.
Schools are also tracking cases and telling people to isolate. A teen who's been to your house and knows it's illegal may not admit to where they have been. So you may not be "officially" told to self isolate. In this circumstance some people will (have to) carry on with their daily life as normal.
Unless the teens are coming over when no-one else is present, you are fully ventilating the rooms and disinfecting all surfaces, then it does represent an increased risk.
That's not to get into whether it is right or wrong, but I think some people's perception of the risk is off, because they are assuming that both their child and the friend would get ill at exactly the same time anyway.