They are.
Monster Energy holds trademark SN 97273630,[0] which covers:
"Downloadable virtual goods in the field of beverages, food, supplements, sports, gaming, music, and apparel"
...among many other things.
[0] (basic search for 97273630, change "field" dropdown to serial number) https://www.uspto.gov/trademarks/search
An interesting side-note -- a company actually has to be using the trademark in commerce in the specified categories in order to qualify for trademark protection. This implies that Monster has products on the market of each type in the list.
I quite agree, which is why I'd expect them to be referred to as fields rather than field.
Suggesting they only have drinks interests is not correct.