The reason STEM graduates can earn more is because there is a higher demand for them - they are core to so many industries, yet we don't have enough people studying these subjects for our needs. This pushed up the amount they can earn.
If we lower the tuition fees for non-STEM subjects and keep STEM at the current rate, that will only encourage people from not studying STEM subjects, increasing the shortage of such graduates.
Instead of solving/helping the situation, it would further increase the salary that STEM graduates could command, while also hitting our economy due to lack of suitably trained staff.
If anything, we need to be aiming for a highly trained STEM workforce, especially in the area of technology, in order to boost our economy. That can be achieved by making it more enticing to study STEM subjects at university - we do that by lowering the fees for STEM subjects - not raising them.