These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternate Character Interpretation: Some have speculated that the frog is deliberately not singing for the man in order to get him in trouble. Some even speculated that the man was hallucinating the frog dancing, but this has obviously been Jossed. The ending to "Another Froggy Evening" suggests that the frog will sing if properly asked.
Broken Base: It's been observed that, like Michigan J. Frog, the cartoon itself only "performs" for certain people.
Technology Marches On: A younger viewer might wonder why the man doesn't just rent a camera and film the frog. 1950s movie cameras were expensive, needed a lot of supplemental lighting, and only gave good results in the hands of a skilled operator. And sound had to be recorded separately. Even if this was made later when cameras were more common and less expensive, it's possible the frog would not perform in front of filming devices anyway, knowing the man's only out to make profit.
The Woobie: Admit it, you felt bad for that guy at some point or another.