imagine you're in a world where most jobs are pretty crappy and you don't really want a job any way the internet is so great why would I go to work for 38 hours a week serving people ham?
imagine you're talented, you make great animations, people love your animations. you make these animations and post them on newgrounds, you make a few hundred $ a month from the advertisement revenue and you can pay for a cool new wacom tablet with the money, or a new mouse, or go to a nice party, but you still need your job to pay rent
imagine a company says "hey, your animations could attract so many more viewers on youtube and you'll make a lot more money per viewer, because we have a huge number of serious advertisers"
"how much more money?" you say
"if you clear a couple of million views a month you can pay rent and just look at the amount of views this has on youtube when compared to it on newgrounds.com" they say
and so the cogs are turning in your head, more views, more money per view? so you're like, "you're saying I can continue what I'm doing with my animations but more people watch them, I have more fans, more opportunities and the extra money means I can quit my job and do this full time? my parents won't think I'm worthless?"
imagine your answer.
think of Newgrounds and Machinima as employers and animators as employees, if you pay peanuts and your employees can get more elsewhere with better benefits why would they stick around? Good will only goes so far... it's unfortunate that the world works this way but money is money and if you can make money doing what you love, exposing your art to more people ('cause that's what creative people love) why wouldn't you?
As an aside it could just as easily be argued that it's unfair of Newgrounds to expect artists to stick around and continue to pay attention to a website that doesn't give them opportunities they can get elsewhere. Think of it like your parents, yes your parents nurtured you for the first 18 years of your life, but after you've grown up should you be expected to pass on opportunities to stick around and help them in old age? Some cultures think yes, others think no.
ultimately it's up to each individual artist but you shouldn't begrudge companies for providing more opportunities, because more opportunities for the artists you love means more content for you.
Machinima aren't stealing artists, they're providing better opportunities.
replace "machinima" with "any Youtube network"