You´re losing your student privileges.
The law then says that your studies are no longer the "main" part of your "life".

Meaning you will have to pay the same things as a normal employee. Including unemployment insurance, health insurance and so on (always fixed percentages of the wage). So you´re set to loose a nice piece of your wage.

(As a student you only pay a fixed amount of money each month for health insurance. Right now I believe around Euro 50 per month.)

And additionally the employer normally pays half of these percentages. Meaning additional costs for him too. So given the chance he probably would like to avoid that. :)

The university itself isn´t interested in what you do.