But how does one who goes for this promotion be "Guaranteed" the site will be up after the bets are placed and then when the event ends you won't just run away with everybodies money?

It is very convenient for a site to blame it being done for "maintenance" reasons while they are not planning to return again and have all members/players funds locked in their accounts!

They can't guarantee that the site will be up. It's quite unlikely that they will disappear though, as I'm betting that across their 3 sites (bitcasino.io and vegascasino.io are owned by the same company), they make quite a bit of profit with those high house edge games, and their high margin sports betting.
If they were going to run, I'm guessing they would have a larger promo (not sure which one you are talking about, but if it's the moneyback offer, they would at least do the 100 mBTC they used to do) as they would have no intention to pay.