The benchmark is incremental. It measures how fast you can insert N rows given M rows already in the database. So, the second data point means you can insert roughly 750,000 rows in 140 seconds, given 750,000 rows already in the database. The limit certainly doesn't approach zero as the number of elements approaches infinity :)

The engine isn't open source. We're considering open sourcing it in the future, but we want to understand all business implications of this decision before we proceed - it's a decision you can't easily retract. The license is a bit draconian, but this is because we've only released a developer pre-alpha. We don't want people to use the engine in production yet - it's not ready. AFAIK, the license says you can't sell RethinkDB support, not that you can't sell support for software that uses RethinkDB.