Acceptable losses and risk management
While Bezos probably drank a whole bottle of pepto-bismol after signing this deal, it fits. Amazon won't want their main competitor profiting off of their launches, so they are willing to fund even experimental launch platforms. Unlike a 30-year com sat out in geo-syncronous orbit, their Kuiper sats have a limited life span and are undergoing rapid development, and they are deploying a large constellation, so they can afford to lose some to launch failures.
What they can't afford is to let SpaceX get so far ahead they are permanently closed out of the market. So they need to reliably get enough of their block one sats up in orbit and generating data for them. SpaceX has a good record and the best launch cadence.
SpaceX will play along, as they still have first mover advantage, and they are probably gunning for a duopoly to keep the regulators at bay. If the new launch platforms deliver, they will be pushing more and bigger sats into their own Starlink constellation, and if Bezo's is on the sidelines when that happens, he may not get a chance to catch up. He can't afford to wait, but will still try to either pay other launch partners or play catch up with New Glen.