Why do free online games like Fortnite need a paid subscription on the Xbox?

Fortnite is free on all the devices it is available. You buy the device and you play the game without every having to pay a single penny more. Except for on the Xbox. Xbox live gold is required for which you have to pay either every year, month, or 3 months. I think Xbox live gold should not be required to play a free game. Other companies like Sony already do this. I think this should also be their for the Xbox.

:)