I don't think its about rewarding films though. It is generally used as a way to judge how well a film will do financially. Films with low cinemascope typically don't do well with audience and so don't make a lot of money. That's usually how the companies judge success after all.
Case in point:
Killing Them Softly (78%) - Domestic Total Gross: $15,026,056 on a budget of $15,000,000
Solaris (65%) - Domestic Total Gross: $14,973,382 on a budget of $47,000,000
Darkness (4%) - Domestic Total Gross: $22,163,442 on a budget of $10,600,000
The Box (45%) - Domestic Total Gross: $15,051,977 on a budget of $30,000,000
The Devil Inside (7%) - Domestic Total Gross: $53,261,944 on a budget of $1,000,000
So out of the films I could find the budget for only two made a profit domestically and one of them just barely.
I haven't seen Solaris, but I have seen and enjoyed Killing Them Softly but I can easily see how it didn't appeal to a wide audience, which is really want CinemaScores is gauging.