Usually it’s just a legal ass-covering, but in this case, the project most likely depends on F4SE and the address library, which injects stuff at specific places in memory. Theoretically a cracked exe could work, but given something is likely changed from the original, all memory pointer addresses those two previous tools hook into and or override could be wrong. It’s less of a piracy thing, and more of a “We don’t want to troubleshoot things that may require us to do illegal things”. Besides, the issue would have to be fixed in the original libraries, not the mod.
Whilst developers can ban specific ads, they can’t really vet them beforehand. I think you can opt out of political ads (or maybe could at one point), but technically the ad you see is just for another app.
As an alternative solution to the ones already posed by others, you can use “Google Opinion Rewards” to get enough play store credit to buy the app. Every so often it’ll just ask if you googled “Ninja Coffee Bar” (literally), and give you 10-25 cents for answering, and typically a good bit more for any follow-up questions (“How helpful were the results”, etc).