Not an admission of guilt?
I don't think we should necessarily see this as an admission of guilt on the Cardinal's part, or a final judgment on the issue by the Pope.
It is not clear that there has yet been a proper investigation of the claims, and he has previously indicated that he rejects them.
It is true of course that we've heard claims of innocence many times before from the guilty, and a reputation as a progressive doesn't help one's cause in this area. Still, the Cardinal is entitled to some chance to have the claims properly tested.
Either way, though, the accusations of four priests (whose decision to go public was perhaps a shocked response to the perceived hypocrisy of the Cardinal's assorted comments) have to be treated as credible on the face of it, necessitating urgent action on the part of the Pope.
The Cardinal was already over 75 and thus had already tendered his resignation and had it accepted. The Pope formally setting the date for its effect was the only way to handle the potential scandal in the circumstances.
The Cardinal problem
Removal from Office as a bishop is one thing though, but why has the Pope hesitated to remove those accused or actually guilty of crimes from the Office of Cardinal?
Perhaps in this particular case the decision not to depose him from Office or force his resignation as a Cardinal reflects the fact that the processes haven't been completed. But there are plenty of cases where the evidence of improper behaviour on the part of men who remain Cardinals is reasonably clear cut (think Cardinal Law, for example, thankfully excluded from voting by virtue of being over 80).
In this particular case the Pope or his representatives may perhaps have gently suggested staying away from the Conclave. Yet O'Brien remains a Cardinal - so, like other controversial Cardinals, he could have chosen to turn up if he wished.
Indeed, the Motu Proprio released by the Pope overnight that allows for an earlier Conclave should the Cardinals agree also reaffirms that no one can exclude a Cardinal-elector who wishes to participate in the conclave from doing so.
Whether or not Cardinal O'Brien is guilty of anything, that he has chosen to stay away in order to avoid further controversy is entirely to his credit.
Would that some others would follow this lead.