I have noted that a few online trading platforms have the following policy for withdrawal: they will send money using the same channel which the user deposit in the past. For example, if I deposit using PayPal, they will send me money using PayPal. I cannot, for example, deposit with a credit card and withdraw my winnings to a bank account, even if I can prove that I own both.
Why is that?
Because if they allow chaining, you can do the following scam:
So if the credit card company awards the chargeback (which they tend to do if the card was stolen), then the platform is out the money. They can't chargeback the PayPal payment, as that money is gone.
If they always put the money back where they got it, then they are never vulnerable. You triggered a chargeback? They point out that they already refunded the money.
This is why if someone deposits money to your PayPal account and then asks you to send the money back, you should just do a refund instead. Because if you send the money back and then they do a chargeback, you can't say, "But I sent the money back in a separate transaction." Or you can, but they'll still take the money from your account.
You might argue that if you could prove that you own both the credit card and the PayPal account, that won't work. That's not entirely true. Consider if a thief steals both a credit card and a PayPal account from the same person. If the thief has access to the victim's mail, the thief might prove that both accounts are the same person. The thief withdraws the money from the PayPal account. Now, the victim finds out about the credit card. The bank reverses all the transactions. The platform is out the money.
Or take the possibility that you deposit the money via credit card and withdraw via PayPal. You do a chargeback. For the next six months, they don't have their money. They eventually fight off the chargeback. But they still lost the use of their money for the six months.
Their credit card processor may even have a requirement that they do things that way. Because the processor doesn't want to be in the middle of a chargeback either. If the processor can see the money going both in and out, then they can answer on their own. If they can't, then they have to go through the effort of collecting more information. And then what if you dispute the additional information? It's not theirs. They don't know whether it is true or not.
I think there's (also?) a different reason than in the accepted answer.
In general, credit card companies don't allow merchants to do cash-equivalent transactions, because they expect those to be done as cash advances.
So if you could deposit with a credit card and then withdraw cash, you'd bypass the implications that come with cash advance (interest fees, limits, etc.).
There is also (or maybe even most important) a legal reason: As soon as a company transfers money from a source to a different place (like it would happen in this case), it fulfills the definition of a bank in many countries like all EU countries. And this requires the company to fulfill all of the strict rules a bank has to fulfill (regarding funds security, money laundering and others).
To protect themselves against consumer fraud and protect their customers from malicious access where a fraudster logs in and then tries to withdraw funds.
Edit : Most firms won't disclose this for safety reasons but I did find reference in the Robin hood FAQs: https://support.robinhood.com/hc/en-us/articles/208650296-Transfers-to-Your-Bank-Withdrawals-
