There are some banks that have blocked pornography and medical marijuana purchases.
But basically, the pattern is clear: banks don't want to lend you money to make investments. Banning money orders is a natural extension of that because you can then go use that cash for the aforementioned things.
That makes a lot of sense, because if banks did want to make those investments, they don't need you. If a bank wants to invest in stocks, property, or a game of roulette, they can just go put their money there, no credit card customer required.
What banks can’t do without lending you money is get a little slice of your paycheck every month through interest payments. This is a separate asset class than things like stocks.