Do they get a commission for accepting and depositing unregistered stock? I would think the enticement to do what they were busted for has to be the money above any in/out transaction fees.
In the etrade case, greed, stupidity, or not knowing the regulations are all possibilities.
On the extremely sleazy end of the brokerage world, there are some brokers who specialize in unloading "problem stock" (read: stock in penny scams). Here is an example of one who got caught:
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