It were okay, but the problem is- the industries that exploit the young and naive - actively pull out tentacles to draw in more of the young and naive. Thats what all the hollywood "how to become a star" and "talentshows" are.. get fresh meat..
That isn't it. Employers know that if you have passion, you'll be willing to work more for less money. Which is why they so often insist on passion.
Also, I've seen plenty of people on HN insist that people doing "creative" work (excepting programmers, who are obviously worth every penny) should do so for free so as not to taint the purity of their work with capitalist incentives.
It isn't an assumption that what's fun can't be highly paid, rather it's an assumption that what's fun shouldn't be highly paid.
You will also get paid less than you are worth and eventually start to hate what you love.