I think we are seriously getting off plot here looking for reasons they haven't cancelled the shares but have put them in treasury.
It could be simply because they are going to cancel the whole lot when they have finished buying back.
On the other hand they could use them if they did another placing but the cost difference between doing it this way and simply printing some more shares I would guess is neglible.
I wonder they could use some of them for directors options?
But if you really want my guess why it's as follow.
SEAM own exactly 20.0% of CWD or 19.99% if you want to be exact. I am guessing their internal rules won't let them own more than 20% of any company. So, if CWD cancel them instead of putting them in treasury for every 10,000 shares that CWD buy SEAM would have to sell 2,000 shares which is slightly counterproductive.