[nolemmings]

This proclamation is certainly after the fact.

"Regardless of the verdict of juries, no player who throws a ball game, no player who undertakes or promises to throw a ball game, no player who sits in confidence with a bunch of crooked ballplayers and gamblers, where the ways and means of throwing a game are discussed and does not promptly tell his club about it, will ever play professional baseball"

I can agree that the notion of placing an affirmative duty on someone to report a fix, as opposed to participating in the conspiracy itself, is something that may not have been articulated well (or at all) before 1919, and thus view Weaver's case somewhat differently if you believe he opted out. Still, before that time players had been been banned for conspiring with gamblers; heck even for contract jumping, so it wasn't a huge stretch to at least fear a lifetime ban if you were known to have "associated with gamblers", even if they were your own teammates.
Remember too that Weaver et al were charged with crimes of conspiracy, which would have effectively ended their careers had they been found guilty and thus in essence imposed a form of banishment apart from baseball's own decision-making. Seems to me if I do something that might be criminal it would occur to me that my profession might take a dim view of it as well, such that I shouldn't be surprised if I am disciplined even if my exact situation might not have arisen before with others. It's a shame it put him in a posiion of ratting out his friends or going down with them, but given the enormity of the scandal and how it could have completely ruined baseball, I can see why Landis took an aggressive approach with the rod.