Why do we pay to present at conferences?
It costs a lot of money to put conferences on.  Generally, the fees are to help cover this.

In many fields, the majority of people attending are also presenting.  If they didn't charge people to present, they wouldn't be able to generate enough revenue from attendees to make it work.

Note that $50 is very cheap for an academic conference--in my experience, most cost hundreds of dollars. Even then, in many cases the conference fees don't cover the whole cost of the conference, and the difference has to be made up with sponsorship or from other sources.