Private companies are - often - not driven by pure greed. The people work hard for their company to be successful and they are much less likely to fuck it all over with a stupid move.
Public companies, however, are controlled by a bunch of money motivated monkeys that couldn't give a rats ass about a company's future. They'll bleed it all dry and go to the next company.
I never understood how that is valves problem tbh. That's like being mad at football players for sports gambling. The second you have a somewhat open market, some people will abuse it.
Also, what's valve supposed to do? They bab the trade bots if they can find them, they have sued the operators of those portals in the past - so what do you realistically expect from them now? Completely close the market just so some people can't gamble anymore?