Remember mayday situation is one in which a vessel, aircraft, vehicle, or person is in grave and imminent danger and requires immediate assistance.
Well, also remember that it is not easy to get a reclearance in oceanic airspace for situations of i.e. weather deviation or unability to maintain altitude.
When reclearance cannot immediatly be obtained, there are procedures in force that include use of pilot's annex 2 "emergency authority".
Captplaystation's explanation seems much more rational then the "panic in the cockpit" one.