Because you can access it for free? As it was the case from the first day LLMs became a thing in public consciousness?
Or did I miss the change, and ChatGPT, Claude and Gemini no longer have a free tier anymore? And then the next tier that costs peanuts for anyone in the west, that gives you more access to slightly fresher models?
That is interacting with it - which people are getting charged more and more for.
The first ones are free.
And as long as they keep the weights proprietary (which all the major players are for their primary models), they can decide to charge whatever they want later.