"Innocent until proven guilty" is a legal phrase. Society is, and businesses are, free to disassociate from whomever it likes. And pragmatically it seems that they have little choice but to cut Spacey loose. Perhaps if he'd denied the charges altogether, rather than essentially owning up to them, then he might have stayed on. But he tried an unusual and underhand, and frankly disgusting, tactic to get loose ("sure, I may have abused kids, but I was drunk and also gay"), and frankly that sort of approach sealed his fate.
It may be difficult to prove the allegations in a court of law, so he may always remain legally innocent, but you don't have to wait for a guilty verdict to feel that keeping someone close is bad for your reputation and profits.