If a student 'dumbs down', it doesn't mean that they're taking an 'easy' course - it means that they're taking a course with a less able candidature. This makes it easier for them to beat other students, and hence to obtain a high ranking in the course.
Imposing a maximum scaled mark, which varies with the demonstrated ability of the candidature, counteracts this.
The maximum scaled mark for an uncapped course is always 100%. There is a procedure currently in place which 'stretches' the distribution of marks for a course if the actual maximum scaled mark is less than the maximum possible scaled mark, such that the maximum possible scaled mark is always attained.
My personal opinion is that if the maximum scaled mark is going to be reduced for courses with 'less able' candidatures, it should be increased for courses with 'more able' candidatures - above 50 if necessary. It's a little unjust otherwise. Don't know whether that will ever happen though.