1) Yes that is correct, the usa tax law demands that any income (as in earning money in a possible way) is needed to be paid in tax.
https://en.wikipedia.org/wiki/International_taxation
2) I can only assume because the website I think is in North America, unclear where the creators live or if the server is in North America.
3) The buyers country info is due in part of EU VAT, which is way different than USA. It is highly recommended you stay in "collect by itch.io payment method" for anyone who does not know about taxes or VAT. All companies have easy ways to help sellers do the taxes because they want you to sell stuff not worry about taxes.
Extra Info It is reported that Steam does not have this feature for reasons we do not know why.
https://partner.steamgames.com/doc/finance/taxfaq
Q. What is my responsibility regarding VAT, sales tax, or other similar taxes?
A. We can only provide you with information on how we handle VAT and sales taxes. You will need to consult with your tax advisors on your own tax obligations. In the monthly Steam Sales Report which can be viewed on the secure partner site (https://partner.steampowered.com/) under the "Reports Tab", you will find a link at the bottom of the report called "Steam Sales by Country". This may assist you in calculating your own tax obligations, if any.
Do not use Steam if you do not understand taxes, you only get in trouble.
4) Yes Ravenmore it is confusing, i'm sure you did the tax right since they also never reply back to me.