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.
Good question. I'm not sure. Given diminishing marginal returns if MIRI and OpenAI are doing the same things then the value of giving to MIRI goes way down. In contrast, if OpenAI is going to speed up the development of AI without putting too much thought into friendly AI, then MIRI and OpenAI are complements and it's more important than ever to give lots of money quickly to MIRI.
Another factor to consider: If AGI is 30+ years away, we're likely to have another "AI winter". Saving money to donate during that winter has some value.