Individual Universities are able to stipulate whatever qualifications they like. A-Levels are generally considered 'higher' academically than AS-Levels, as they require a greater depth of study. Obtaining 320 points with three A-Levels, for example, is more difficult than obtaining the same number of points with 16 grade E AS-Levels.
If a University finds that a particular course is over-subscribed then they are well within their rights to show a preference for A-Levels over AS-Levels, just as they are able to show preferences for certain subject combinations.