What makes intentional low or missed approaches legal?
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.