Honestly, the answers that the Miso founders gave seemed very professional and I think am a bit on their side in this one. If OP had asked for the bonus while still being at Miso, it seems like he would simply have gotten it. Seeing as a non-contractual bonus is an investment in the employee that the company does in addition to the usual compensation, and seeing as OP quit on his own and doesn't have a compensation a…
I tend to agree with @rb2k. There are payments you are entitled to by law (salary, vacation, personal expense reimbursement) and payments that companies do out of the goodness of their hearts (annual bonuses, lieu days, extra days off, referral bonuses). Don't quit under bad circumstances and expect to get money you're not by law entitled to. Should have gotten the money first.
1) Yes, a referral bonus is optional
2) It no longer becomes optional once you make the offer to your employees. At this point you have established intent; especially if it is provided in writing. You cannot take it back once the other party involved has upheld their end of the arrangement.