How do you envision them stealing from someone’s balance?
The big mining ops actually have more inventive than I do to see that the currency isn’t devalued, as such things threaten their investments.
My understanding is that the blockchain doesn’t allow for instant transactions. Because of this transactions have to be prioritized and priorities are usually determined by who is the biggest stakeholder, i.e. who has the most Bitcoin. This allows for pumo and dump operations as tIe pumpers can get out before the suckers.