My interpretation is that this is to enable Greensmith to do the full season more cheaply by only paying for the drive and for not sponsorship as well.
Castrol has had a very long-term deal with M-Sport and is an important source of money not to be pushed away for a short-term deal.
It's also doubtful that Crown Oil would get much payback by sponsoring the WRC team. They already sponsor local football teams and stadiums. I'm sure this brings in more sales of domestic/industrial oil for heating than the more niche sport of rallying.

