Fuel is made from a range of hydrocarbon components (about 2000 in gasoline) each of which has a different boiling range, but also different combustion properties. The proportions of these components blended into the fuel is called the formulation. The formulation varies from fuel to fuel, country to country and even in some countries from season to season. The 'lighter' components, for example butane and pentane evaporate quite easily and are usually inclused in the formulation to help with cold starting. As Woolly says, some of the fuel i.e. the heavier components don't evaporate when the engine is cold, because their boiling point is too high for the cold conditions. So as only some of the fuel is made up of the lighter components, during a cold start more fuel is injected to compensate for the fact that some of the fuel won't fully evaporate. Liquid fuel doesn't burn of course.
With drive-by-wire throttle control, not all systems have by-pass air valves today. Essentially the ECU can just open the throttle a tad to allow a faster idle speed when the engine first starts up. This helps raise the exhaust temperature which in turns helps to warm the catalyst up so that it can manage the unburned hydrocarbons and CO that result from running rich for the cold start. So your point about emissions is a good one.
About 10 years ago in Europe, during an emissions test the engine was alloiwed to idle for 40 seconds before the exhaust gases were collected, followed by another 11 seconds before driving away. Today the exhaust gases are sampled from the moment the engine is started. This makes quite a difference to what the engineers can do with the cold start strategy. Also the test requires the car to start moving after 11 seconds, so no need to idle for a significant period before driving away.
To comply with US regulations, the car's emissions are assessed using the FTP75 drive cycle. I'm not as familiar with this cycle as the European one which is called the NEDC. I think it's fair to say that while 'emissions cycle beating' ECU mapping is not allowed, it would be reasonable to expect MB to map the ECU to match the market into which the car is being sold. Not
only are the emissions cycles different, but the fuels and oils are different across the globe too.
Anyway, blathered on enough. Hope that was helpful in some way..