In general the lounge load on Saturdays is very low. Also it is not a travel intense day for business people. So why not redirect some load and attract silver members to be more loyal with SAS?
The costs for SAS are almost exactly the same ( as the lounge is manned anyway) and it is a nice perk for not so frequent flyers such as silver members.
I remember that I would have appreciated this feature when I was still silver ( meanwhile I am EBD)