The Democratic party is really not required to have any sort of fair process for deciding their candidate for president. It can be as transparently crooked and disgusting as this and really that's just how he chose to operate within the legal frameworks established by folks like him.
Sure, there's the huge personality flaws that Bloomberg displays by taking this strategy, but those don't seem to be of major concern to most American voters. Our current president is a daily salient reminder of this fact.
If we want to focus on Bloomberg's personality, why not start with all the horrific policies he enacted as mayor? There are far more visible character flaws in those decisions than this decision to buy his way towards the Democratic party.
If you supported his policies, after all, you'd have no problem with this strategy being undertaken - right?