When you invest money in a company, you do so with the understanding that the money becomes the company's and will be spent. In return, you get equity (typically). You don't have an "account" with the company from which you can make withdrawals.
By contrast, poker sites operate like banks, taking deposits from people with the understanding that those deposits will be held in trust. You're not buying equity in the poker site, and your money doesn't become their money. It's still your money; it's just being held in an account they manage. This is why commingling is bad--it obliterates the distinction between their money and yours.
Obviously, these poker sites are not FDIC insured like banks. But a legitimate poker site would still take all reasonable precautions (such as segregating the company's funds from depositors') to protect your money.