Because, on a commercial flight the dual controls must be removed unless the seat is occupied by an appropriately rated pilot. Alternatively, being left empty.
Might that be a company requirement, rather than a legal requirement?
However, CAA FODCOM 21/2010 gives more guidance on the interpretation of the rules.
Note that the rules apply to public transport flights.
www.caa.co.uk/docs/33/FOD201021.pdf