It is not that simple.
Calories live very well in a lot of countries which have otherwise switched to SI units. Here in Denmark we are very much SI, but our food packagings still have nutrition information…
Yeah, when I lived in the UK and visited the doctor for the first time, they asked me for the weight, so I replied in kg (note: officially the UK uses the SI-system since the 80'ies). So they had no idea what it meant, reluctantly they asked "...in stones...?" - I thought it was a joke and asked "... yeah ... and pebbles!?"
In the end I learned that the UK still was stuck in the stone-age. - This can also be seen on their maps: using metres for elevations and distances in land-miles. (not even nautical ones, which would make sense, somehow) WTF?
But the US is no "inch"(=thumb-width) better - using a system which has no definitions, but is defined by the metric system (1 inch =25,4mm is the official definition!). It is so strange to see that they destroy satellites worth Giga-US-dollars or crash planes, because they cannot stick to a well defined and standardized set of simple decimal rules that do not require weird conversion factors like "the furlong ... is equal to one eighth of a mile, equivalent to 660 feet, 220 yards, 40 rods, 10 chains or approximately 201 meters". 
- On the other hand it is fun to see that the first thing ongoing scientists have to learn in the UK and US is the metric system. - I wonder whether this might be one reason for the success of foreigners in these countries.
Therefore, Garmin please forget about the kcal and abolish the mph and km/h* as well, to replace them by speeds humans can interpret like m/s - and make the local semi-standards only available for indigenous people as an option.
Thanks! 
* using hours with 3600s is a strange remanescense of the Roman duodecimal system and thousands of seconds are hard to grasp for athletes anyway.
I added kilojoules to Garmin Connect Web using GCOverrides:
