Firstly, we can't tell that Heatran is a frog. According to Bulbapedia, it's origin are:
Heatran may be based on volcanic remains. Its Steel typing may reference how the Earth's inner core is made of solid metal under immense pressure. It may be also based on a generic volcano monster: in the ancient mythologies of most cultures, the eruptions of volcanoes were explained by the presence of a fire monster living within it, often a dragon.
Not sure that they distribute Bug Bite to Pokemon that are bugs/insects. While non-bug types such as Heatmor and Trapinch which get it via level-up and breeding, Heatmor is an anteater which are mammals close to sloths, they aren't insect. Second, Trapinch is based on antlion which is of insect family.
But via breeding, there are also many non-bug types too, such as Victreebel line, Carnivine, and Gliscor line. The former two are insectivorous plants which eat insects, and can learn Bug Bite. Gliscor is based on scorpion, which is of insect line, so it makes sense.
The distribution of Bug Bite instead of being limited to only insects is also given to those Pokemon which are based on creatures that bite or have fangs, such as Gliscor line, Trapinch line, Victreebel and Carnivine line.
We can conclude that Heatran isn't a frog (guaranteed not though), and Bug Bite distribution isn't in such way.
Hope this helps!