I think the reason why pinterest did this was as a vanguard of what you see here in the article now. There are still many companies that are in pinterest's position and do not do what they did.

The engineer market in SF right now is very highly heated, and this kind of incentive makes an offer from pinterest far more appealing. That and people are starting to compare notes and make articles on medium and others really spelling it out is probably hurting recruiting and this is coming out as a response.