Is it ethical for a lecturer to require students to purchase an online learning kit where kit is used for course assessments?
I'll start with 


  How may I fight for our rights as students on this issue?


For one, politely express your displeasure to the professor. 

Some professors aren't in tune with the cost of textbooks, and may not realize what a burden it is to shell out $90 for their course, when it's $90 that you can't recoup by selling back the text, or avoid by using the library's copy of the text.

They may even think they're doing you a favor by offering this "interactive" system, not realizing that you don't find it useful.

If the professor isn't interested in hearing what you have to say, you can further express your displeasure in the end-of-course evaluation (my university's specifically ask about the textbook) and possibly in a respectful letter (signed by your classmates) to the department chair.


  Is this ethical?  Wait, is it even legal? 


As the other answers have pointed out, it's not universally considered unethical or illegal. 

As unpleasant as the result is in this case, it probably would not be a good thing to set too many bureaucratic rules on what textbook or other educational resources a professor can assign. Given that the cost of tuition is usually many times higher than the textbook price, it is generally desirable for the professor to assign the textbook that will (in his/her opinion) offer you the best educational experience and therefore, the best return on your tuition investment.

Of course, students tend to appreciate when the professor can find a low-cost option that is also educationally sound. But that's generally considered a bonus, not a requirement.

In this case it sounds like the extra content doesn't contribute anything useful to the educational experience - in which case, you should let the professor know. (Again, politely and non-combatively.)

I also want to point out that requiring "bundled" course content that goes along with the textbook is closer to assigning a required software package (which, unlike textbooks, you can't buy used or re-sell) than assigning a textbook. I agree with you completely that it's substantially different than being asked to pay for a textbook. 

For every class I've ever taken that required specialized (non-free) software, my university installed the software on my laptop for me for free (given that I was registered for the course in question). In most cases, instructors try to use free software or software that comes with a free student license, because they realize that a software purchase is different from a textbook purchase. Or they'll provide the software on lab computers that students can use during set hours.

Imagine if you had to buy a new non-refundable software package for almost every class you were enrolled in, including buying the same software multiple times for a sequence of intro classes on a subject.

Yeah, it's a waste of money, but your professors might not see that unless you point it out.