Plus, Americans often choose T-shirts (or even tattoo designs) with kanji they think look cool, despite having no idea what they actually mean. Hence the “random” part.
Of course, it works the other way, too. Shirts with English words and phrases are popular in Japan, even though very few Japanese speak or read English. As a result, the words are often random, nonsensical, or wildly inappropriate. (My favorite: I once saw a picture of a beaming woman standing beside her eight year old son, who was proudly showing off his new “Too drunk to f***” T-shirt.)