From their site:
OpenAI is a non-profit artificial intelligence research company. Our goal is to advance digital intelligence in the way that is most likely to benefit humanity as a whole, unconstrained by a need to generate financial return.
The money quote is at the end, literally—$1B in committed funding from some of the usual suspects.
In practice MIRI is more think-tank than research organization. AFAIK MIRI doesn't even yet claim to have a clear research agenda that leads to practical safe AGI. Their research is more abstract/theoretical/pie in the sky and much harder to measure. Given that numerous AI safety think tanks already now exist, creating a new actual research org non-profit makes sense - it fills in an empty niche. Creating a fresh structure gives the organizers/founders more control and allows them to staff it with researchers they believe in.