I think Kuma saved the crew because he was impressed with Luffy and the crew, and because Luffy is Dragon's son. Kuma knows Dragon, Garp, and Sengoku personally. I wouldn't be surprised if Kuma teleported Luffy to Garp or Dragon.

Also, we can't forget the Ace and Blackbeard backstory. The day Ace was defeated is supposed to be the day that triggered a world incident. Kuma is plausibly at or near the center of that story. (D.-family friend, hates Blackbeard, Shichibukai, the raid against the Yonkyou, etc)