"1 month ROI" implies doubling 12 times in a year even with only compounding once per month which is like 4096x or potential 400000% return per year without ever buying a single card from another player. If that doesn't count as "free money" then I don't know what does.
Also, people shouldn't be looking for "ROI" on their $10 spellbook... that's the entire problem.
Again, I agree, even with that insane 'ROI', the scale of that gain is minuscule even if it is 'free money'.
Why do you think so many new players joined up in the first place? For some, even a 39 cent gain per day is better than nothing, which is what those honest players are getting now.