Please provide more details about the offer.

If there's no other information aside from getting 1 ETH just for buying ETH, then obviously it's a scam.

But if there's a footnote or sort of terms about how to get that 1 ETH for buying ETH, then likely it's a promotional activity, and there's a certain task to become eligible for it. Maybe a competition with a prize pool or any promotions close to it.