Well, an association is basically 2-way, at least that’s what I pick up from 22.214.171.124:
An Association declares that there can be links between instances whose types conform to or implement the associated
types. A link is a tuple with one value for each memberEnd of the Association, where each value is an instance whose
type conforms to or implements the type at the end.
But even more interesting is that this behavior is in full compliancy with the UML specification. Check 11.5.4:
In practice, it is often convenient to suppress some of the arrows and crosses that signify navigability of association
ends. A conforming tool may provide various options for showing navigation arrows and crosses. As with dot notation,
these options apply at the level of complete diagrams.
· Show all arrows and crosses. Navigation and its absence are made completely explicit.
· Suppress all arrows and crosses. No inference can be drawn about navigation.
· Suppress all crosses. Suppress arrows for Associations with navigability in both directions, and show arrows
only for Associations with one-way navigability. In this case, the two-way navigability cannot be distinguished
from situations where there is no navigation at all; however, the latter case occurs rarely in practice.
So as far as I can tell this is all in full compliancy with the specification.