There have been a few threads about bus travel in the Budget Travel forum.

As to the OP's specific question, I think that one will be limited to those cards which have a bonus-points category for "travel" or "transportation." But even then, one should investigate whether the particular card issuer classifies bus travel in the bonus category.

As to Amtrak buses, although there are a few exceptions, mostly they can only be purchased in conjunction with an Amtrak ticket, not as a stand-alone segment.