Because they compete with Red Hat and want to steal their customers. And now they can claim they have a superior product because they are using a modern kernel. Red Hat needs to release RHEL 6 ASAP.

To paraphrase some brand of beer, the difference is breakability.

It's nonsense and exactly what branding is for, a linux kernel is a commodity with a price near zero, in order to differentiate a commodity product you need to brand it.

Branding is about perceived differences and having the consumer identify with your brand. Not that you are repackaging the same goods as your competitor, anyone who knows that is already lost to your campaign and as such is irrelevant.