I'm a sophomore in college, and during finals week of the Spring term I got sick and couldn't get out of bed. After requesting an Incomplete Work contract to my English professor, she agreed to let me submit a 2-page long reflection paper after the due date only to, later on, write me a second email saying I should write a 4-page paper. She argued that since we didn't have a final exam and a presentation of said paper was required, it was only "fair" that I wrote a longer essay. My question: is this even legal? I believe what the professor did is not ethical. I feel like she's trying to teach me a lesson the hard way. Another thing worth mentioning is that I went to the head of the department and said that the very same professor was playing favorites. So, I assume she initially gave me a chance with the contract out of fear since she's currently seeking tenure within the English department.
We don't have any idea what the laws are where you are, so there's no way to know whether this is "legal". I very much doubt that it's against any law, though, or even any university regulation.
If other students were required to give a presentation on their paper, then it's reasonable to expect that you also do something beyond simply writing the paper. You could ask that you be allowed to give a similar presentation directly to the professor, though that's still not quite the same, since presenting to a class is a different experience and would generate a different level of discussion. It also presents some logistical problems since you and the professor may not be on campus during the summer. So I think it's reasonable for the professor to ask you to write a longer paper instead.
For a college sophomore, writing an extra two pages is a pretty minimal assignment - by now you should be accustomed to doing much more writing on a regular basis. If she'd said twenty pages, I might have agreed with your "teach you a lesson" theory, but this is really too minor. While you might be able to get a different assignment if you really push, I think it will be a lot less trouble to just write the paper.
I really doubt that this has anything to do with the professor's tenure case. An issue like whether one student has to write two pages more or less is far too trivial to be a factor in a tenure case, which is about the professor's total academic accomplishments over 5-6 years.