Energy does go into the cup, often from the heated food through conduction. However, some plastics can absorb microwave energy too. In fact, that used to be a rule-of-thumb for the plastics company I used to work for with regard to RF welding of plastic packaging.

Oh, and IIRC, most microwave oven's can't reduce the actual power. What happens, when you set it to less than max power, is the unit turns off the magnatron intermittently so as to let the food rest and let the heat spread out through the food. Thus, the food seems to warm up more slowly and with less "cooking".

I like to combine microwave and oven/stove cooking for maximum effect in minimum time. I can drastically reduce the cooking time of some items by steaming them in the microwave, or by partially heating them then browning them in the oven.

The next thing I want to try is an induction cooktop. They're expensive in the US, if you get an integrated model, but portable induction cooktops are common in asia and fairly inexpensive by mail order, or so I hear. I so want to boil water in 90 seconds. I can never get the water temperature quite right for tea by microwave, and an the electric stove takes too long and is too inefficient.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau