When did I suggest they wouldn't want to work in their subject? The problem is that some degrees aren't as valuable as others. A maths graduate has far, far better employment prospects than an English graduate, for example, so a maths degree is more valuable to the individual, but both are equally valuable to society.
These changes encourage people only to pick degrees that will lead to personal financial benefits.
So the Maths degree should have the more expensive tuition fee because it's more valuable than an English degree.