I don't know where you get that idea. They explicitly say "some Tweets from our test partners will feature a “Buy” button", there is no mention of ads. The implication is, if I don't follow their partners, I won't see the buy button.
Then any corporate accounts you follow now are 'ads'. It's an added button, no more, no less. If that magically makes an otherwise neutral tweet an ad, you need a more rigorous definition of advertisement.
That's a possibility, but that still wouldn't be more intrusive than the current sponsored tweets. If anything, if this model is successful, they might even reduce the amount of sponsored stuff appearing in your timeline.
