...that Saudi monarchy is performing a balancing act. They'd rather not have to deal with pissed off American's or with pissed off Wahabi's. Both are a danger to their lifestyle. If you understand that they are into appeasement, you'd quickly figure out why they take actions which are seemingly paradoxical.
This same balancing act hasn't gone away, though 9/11 did require a shift in Saudi strategy. Since you point out the arrest of this "Big Fish", have you also not taken not that there was an issuance of an amnesty for those that turned themselves in? You may find out that the catch and amnesty are interrelated events. Then again, maybe not.
Maybe it's just the latest effort of the Saudi's to placate the Americans. Next turn will be to placate the fundamentalists. It's a never-ending balancing act, that may not have a happy ending, as there is no semblance of equalibrium in the equation.