huh?

Because the IRS has absolutely no idea where that money came from.
All they are going to see is a huge amount of money go into your bank account (maybe).
It is expected that you might be required to report on this deposit.
In fact you might even consider telling your bank what it is in advance because they're going to want to know.

I mean I put 1 BTC and take a 30K USD loan, how I be sure IRS will not tax that money when I buy a car or something?

There could be a sales tax on the car but not income tax because it's not income.
It is up to the individual to pay their taxes correctly.
Could a huge deposit like this throw up a red-flag and get you audited?
Yes.
But that doesn't change the fact that it's untaxable.

Thorchain is a company to offer loans in a traditional bank do?

A loan is a loan.

If your friend gives you a loan do you worry about the IRS trying to tax it?
There is no rule that says only banks can give loans.

This is a secured loan meaning it has collateral backing it.
Not much different than any other secured loan.