They don't "have to" give anything back, but the idea is that they are acting altruistically. They feel that the public has supported their company (via usage, interest in the IPO, jobs, income, etc), that they recipricolly owe something substantial to the consumer.

Read a little Ayn Rand (like Atlas Shrugged) for the concept... Essentially, corporations (whether private or not) derive their status as a legal "person" from implicit trust the public has placed in them. As such, corporations have an obligation to provide service back to the community and to act in a moral fashon... I for one am glad to see that this company is acting in this accord, instead of all of the depravity we have seen as of late.

-jokerghost