Corporations are made of people, many of whom have a moral compass, and operate by that moral compass. If you believe in the mission of a company, the company is filled with people who also believe in that mission, and you're making real progress on that mission...then why wouldn't you be passionate?

Sure, you could be "exploited", but many people don't really care whether or not they're being exploited, if they're being treated well and doing meaningful work with people they like -- even more so if they're working towards a goal that can only be accomplished by a larger organization.

Speculating that it's perceived risk mitigation. If you're perceived as loyal, helpful, and even somewhat capable, they'll keep you around! This is in fact the perfect role for that person who gets a lot of juice out of supporting someone else achieve their goals. It is also probably a personality thing.

Of course, you have to answer a generalization of your own question: why be passionate about anything? It doesn't seem to be a necessary, or indeed very useful, to improve darwinian fitness.

This is quite evident in the entertainment (video games, film, music, etc.) industry. It has been established that many creative resources go to companies that produce most of their favorite work.

Sadly, it is quite common for those companies to have rather diverse and relaxed ethical guidelines around crunch and overtime.

Luckily, my own experience is moving away from a company with crunch and a product I didn't particularly care for to a company strongly against crunch making a whole array of games I really love.

Rather: Hiring based on passion is a way to get away with demanding a lot of overtime or crunch time. :-(