101 is, indeed, "guidelines". It is not legal requirements that are set in stone. The recommended maximum altitude for UAS flight, 400 feet, was determined to be a workable compromise between (1) absolutely prohibiting UAS flights, and (2) allowing UAS flights to be as unrestricted as good safety practices can allow without putting other aircraft in danger. Air Traffic Controllers are required to keep airborne aircraft separated vertically by a minimum of 1,000 feet. Manned aircraft pilots are required to fly no lower that 1,500 feet above people, structures, or populated areas (all on the ground) while flying in uncontrolled airspace (take-off and landing zones are considered controlled airspace, by the way, whether there is a control tower nearby or not).
So a drone flying at 400 feet above ground level (AGL) would be 1,100 feet below an aircraft flying at 1,500 feet AGL - easily meeting the ATC's 1,000-foot-vertical-separation requirement - no safety hazard involved. The "extra" 100-feet of clearance between the two aircraft was purposely put in there to account for inadvertent altitude overshoots and undetected equipment malfunctions where the UAS pilot is not receiving correct altitude readout from the airborne UAS.
Another factor concerning the 400-foot maximum altitude "recommendation" is that, at that altitude, many hobbyist drones are hard to even see, let alone to be able to determine which part is the front, the rear, or the sides. Not having that information available from visual reference only can lead to the operator making wrong stick inputs, which can cause some serious problems in many cases.