Airline 'X', member of SkyTeam, is offering a full Y (Economy unrestricted) fare on a certain route, which comes with the following rule:

fl.flt application
the fare component must be on
one or more of the following
any 'X' flight operated by 'X'
any 'Y' flight operated by 'Y'
any 'Y' flight operated by 'Y1'
any 'Y' flight operated by 'Y2'
(and this is all)

Airline 'Y' and its subsidiaries 'Y1' and 'Y2' are not SkyTeam, but Star Alliance members.

Strange or not? Please enlighten me