Onedrive for Business can't connect to subsite

I do have a question about using Onedrive for Business with Sharepoint 2013.

I have the following Sharepoint architecture:

http://ift.tt/1Mgec02

Where site1 is a SP site collection.

Now I created a subsites for site1 (site2) and another subsite for site2 (site3) http://ift.tt/16XSTzJ

Some users should get rights for site3 but not for site1.

Now if I want to sync files with onedrive for site3 I get the message that Onedrive can not connect to the site3. When I give the particular user the right "site1 visitor", it works.

Why is that? Isn't that a normal use case?