They probably think of it in terms of control. One of their big upsells is controlling access to shares. They don't see it as a problem that you have to save it to the device then share from that because it's not for you from their perspective. It's for people sending project assets to collaborators.
I think it makes a lot sense for the dropbox link to be the default, especially on mobile.
I almost never want to download the file over LTE to my smartphone and re-upload it to a different web service and waste (2 * filesize) of my limited mobile bandwidth to share a file.
