What key is "Dreams" by Fleetwood Mac actually in?

It's not F Lydian

Here's a simple-seeming song that is a subject of a lot of music-theoretic controversy. "Dreams" by Fleetwood Mac only has two chords (plus a third chord that only appears once), so it seems like it would be easy to analyze its harmony. And yet, no one can agree what key it's in.

The two chords are F and G, repeating endlessly for one bar each. During the choruses, you could call them Fmaj7 and G6. There's also that lone Am chord in the instrumental break after the first chorus. The vocal melody is all on the A minor pentatonic scale. That would seem to settle it: the song is in A minor. So why all the controversy?