What makes intentional low or missed approaches legal? A common procedure in flight training is the low approach: simulating a landing, but without actually making contact with the ground before powering up and either going around in the traffic pattern or executing a missed approach procedure, sometimes after flying all the way down to an approach's minimums.
In this situation, a pilot flies very close to the ground - one suggested crosswind training procedure suggests flying along the runway low enough that &quot;[i]t's OK if the wheels inadvertently touch down during the exercise.&quot;
Contrast this to other operations, including a simulated engine-out procedure away from an airfield. These are governed by FAR 91.119:

Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

Training for missed approaches and going around are important for basic skills, and they are performed very frequently. However, the regulations make no exception for training. For a maneuver where a pilot descends below 500 feet AGL without intending to land, is there legal guidance that makes this OK near a runway, or is it an unwritten exception?
Well, I'm going with "It's a written exception, but nobody thought to include it in 14 CFR 91.119 though".

I say this because there are a number of regulations that require a student pilot or applicant for a license to receive and log flight training for go-arounds.

For example:


14 CFR 61.87 - Solo requirements for student pilots
14 CFR 61.98 - Flight proficiency (Recreational Pilots)
14 CFR 61.107 - Flight proficiency (Private Pilots)
And others


The complete text of the private pilot SEL requirement (and they are all similar) is:


  §61.107   Flight proficiency.   
  
  (a) General. A person who applies for a
  private pilot certificate must receive and log ground and flight
  training from an authorized instructor on the areas of operation of
  this section that apply to the aircraft category and class rating
  sought.
  
  (b) Areas of operation. 
  
  (1) For an airplane category rating with a single-engine class rating:
  
  (i) Preflight preparation;
  
  (ii) Preflight procedures;
  
  (iii) Airport and seaplane base operations;
  
  (iv) Takeoffs, landings, and go-arounds;
  
  (v) Performance maneuvers;
  
  (vi) Ground reference maneuvers;
  
  (vii) Navigation;
  
  (viii) Slow flight and stalls;
  
  (ix) Basic instrument maneuvers;
  
  (x) Emergency operations;
  
  (xi) Night operations, except as provided in §61.110 of this part; and
  
  (xii) Postflight procedures.