I'm bothered by this because Vim is free, with only a hopeful plea for a donation to a charity for helping Uganda as the "payment." It feels a tad unethical to be charging for a slightly more fancy, maybe kinda gamified, vimtutor. Like, just make a vim plugin that does this in vim and let that be your community donation. But I suppose business is business eh.
Making something paid vs. free can be the difference between it existing and not existing. I'd rather have this exist and be a paid app than not exist and be free.