Because there is nothing you can do when a fleet of 10 orcas warps on your moon belt less than 5 min after you detonated it.
Let’s say that in that corp there are only a third of players ( so 17 players and 33 alts). Most of the alts are not fit for fight, can’t bump, or just can’t login at the same time the main in on the field.
What is the owner’s corp supposed to do ? Have 10 of the accounts bump the orcas ? If it’s to get half the yield you can as well mine normal ore it will be worth more.
And again what is the risk for the ninja miners ? None.