To be honest, they're hardly going to buy 20% to then sell 100% of bmi are they?....
For any airline that wishes to integrate some or all of bmi's operations into its own business, it will want full control over whatever it purchases, and will not want SAS 'leaning over its shoulder' asking various questions and requiring disclosure of commercial information to what may be a competitor airline.
By owning 100% of bmi, bmi becomes rather more attractive to any company which wishes to bid for bmi either in whole or in part - the obligation for any bidder to co-operate with SAS no longer exists.
This does not mean that Lufty will sell bmi - just that they are removing one significant constraint from decisions they will make with regards to bmi's future. Of course, the offers Lufty may get from BA / Virgin / Flybe / others may well mean that Lufty decide to keep bmi