Which part is the "theft". They existed to make AI and behold, they did AI.
You can argue they didn't open-source it, as their name suggests, but they also were denied a billion of funding that Musk kept promising until they were literally out of money.
What they did was a matter of survival. Of course they're not blameless in their actions now. They're clearly building a very powerful business. But they were put in this position. It was either this, or OpenAI wouldn't exist.