why is vim autoindenting even if set noautoindent is on?

I type "set noai" to disable auto-indents, but when I have, for example, a piece of code like the following:

int main() {
}

If I put my cursor on the end of the line on the first curly brace, type "i" to go into Insert mode, and then hit return, it still indents with a tab character. Why is this happening if auto-indent is turned off?

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