I hit on some of this in the Reddit AMA that happened in 2015[1], but that AMA was asking the more general question about losing people, not the more specific question about losing people to Oracle. The answer is that it was of course gutting and surprising, and it took a while to process -- it was hard to not read it as a personal rebuke! But of course, people don't generally alter their career trajectory to spite others (that is, you don't hear "Yeah, my new job is a terrible team solving an uninteresting problem within an awful company -- but boy did I ever stick it to my former boss!"), but rather to seek their own opportunity.
So I came to accept that each of them (and there were quite a few!) left for the opportunity they saw at Oracle. As for why Oracle: at the time, they were building a de novo cloud, and they were doing it by hiring engineers away from other cloud providers, with a sharp focus on compensation. And in particular: for the ex-AWS'ers at Joyent, one of their former senior colleagues at AWS (Don Johnson) had gone to Oracle to take a leadership position[2]; I think it's fair to say that those who went to Oracle saw themselves as joining Don as much (if not more) than they were joining Oracle.
Of course, none of this made it any easier for me or for the team to swallow -- and it has been of little solace that those who left for Oracle have (of course!) discovered that Oracle is what it is, with several of them having now left Oracle for other opportunities[3]. They may have learned about themselves what Oracle taught me about myself: that in addition to the team I'm on and the problem I'm solving, the company's larger mission is, in fact, important to me -- and that Oracle's emptiness in this regard can be crushingly soul-depleting.
[1] https://www.reddit.com/r/IAmA/comments/31ny87/i_am_the_cto_o...
[2] https://www.geekwire.com/2014/oracle-hire-100-engineers-seat...
[3] https://twitter.com/mcavage/status/969762887530037248