Assigning record to user from different BU

Hi,

In CRM 2015, a record owner of a BU is able to assign the record to any user from a different BU. These 2 BUs are child BUs of the parent BU and have no other relationship.  There is no common access/owner team as well.  I am aware that 'share' works in this way but never knew about 'assign'.  

This doesn't seem right from a security perspective. Can someone please explain this behavior with any reference link. I wasn't able to find it.