I guess it prevents someone from wandering into the AWS datacenter and grabbing a hard drive, but that seems very unlikely, and presumably anyone with access like that could also get the AES keys, wherever they're stored.
Gilles' comment effectively answers your question, really, but I'll go with a longer answer myself because I'm nice. Disk encryption protects you against data loss when a disk is stolen and the key is not stolen with it. Such examples might be, as Gilles says, stolen backups, but could also be in laptops on the move, or disposed of hard disks to prevent meaningful attempts at salvaging data from your decommissioned disks.
Disk encryption doesn't do much to help you when you put the key and the disk together, because the security relies on the key and if the key can be intercepted, the data can be decrypted. The key and the disk are always in close proximity by necessity when the OS is on and using the disk (every read requires that key) so anyone near it who can reasonably intercept the key should be able to read the data. Of course, you do need to be able to recover the key to effect any kind of attack, so it is slightly harder than just copying a hard disk (but not by much). So basically, yes, you're right.
However, it is still a good idea to protect your disks to minimise the potential loss of data through things like theft and disk disposal. You don't know what or how Amazon do to destroy those disks, so if you have valuable information on there of any kind, having them encrypted is a great idea.
So what's the point, really? Just to say the data is "encrypted"?
That is actually a possible factor. As I say there are tangible benefits from encrypting data are not quite those you might expect, but still exist. That said, I have had customer requirements that data be encrypted on the server end in a similar scenario as a marketing point (we encrypt your data). I think there's an educational challenge there for security people.