Yobit is a reputed and old exchange site.
It's one of the oldest exchanges, but it's hardly "reputed". There have been more scam accusations leveled against Yobit on bitcointalk than probably any other exchange other than the ones that actually pulled exit scams, e.g., Mt. Gox and Cryptsy.
As far as this coin-pumping scheme goes, I don't think any exchange ought to be in the business of doing anything like this. Never. Can you imagine if a stock brokerage like TD Ameritrade or E-Trade or one of those decided to pick penny stocks at random and manipulate them in a pump & dump? For one thing it's illegal, but for another it's unethical for an exchange to screw with what should be a free market. What if you're someone who owns one of the coins Yobit is playing around with and didn't plan to sell it? You might be very pissed off that an exchange is causing the price to go haywire--I know I would be.
So no, I don't think what Yobit is trying to do here is proper. But it doesn't surprise me at all that they're the ones who came up with this hare-brained idea. They don't seem to give a shit about their customers and apparently not about free markets either.