[smeg]

>Google specifically enumerates the rights that you're granting them

They sure do. It just turns out one of these rights is that they can use your content for promotional purposes (advertising?).

Dropbox may not have such a detailed TOS, but they are clear on one thing: whatever rights you do grant them are solely for the provision of the service to you.

[cube13]

The problem is that Dropbox may, at their sole discretion, decide that providing ads is part of "the Service". Under their TOS, they could do that without changing it.

Realistically, Dropbox won't ever do that. But the key thing here is that Dropbox's TOS does not explicitly state exactly what rights they believe they need to operate with.