As exchange for their otherwise unpaid service from those devices, they're using a fraction of your potentially capped internet if you happen to be in a market with such poor competition and/or poor regulatory environment that your service is capped.
I paid $200 (although much cheaper now) for my echo years ago. This isn’t a free device and the services it provides cost me actual money to be able to use. Pretty funny to spin this as “you paid $200 for this device, but all the services on it are free.”
Whether it’s capped or not is irrelevant. It’s still taking advantage of a paid resource without explicit permission. Also if it leads to increased demand across an ISP it will inevitably cause higher prices.
Would love to see a source to back this number up if you have one. I had caps on almost every ISP I had in Europe, but even out in the boonies here in the US I've only seen caps on one (unfortunate) connection.