(They do help clubs sell things, taking "7% of income", so they do have a revenue stream, but the money that Slack wants would pay a veritable army of student interns.)
1) They should know that this is unaffordable for a nonprofit like this. By doing this, they will almost certainly lose them and their thousands of aspiring teenage developers as users. The chance of actually booking that 200K are next to 0.
2) Microsoft learned a long time ago the value of getting young developers using your software to learn. Once those teens start working, maybe starting their own companies or choosing which tools to use at their future empoyers, if they know Slack they are very likely to pick Slack. This is a very short sighted shakedown attempt that wont work in the short term but will drive people away in the medium term.