It's well known that white people who claim to love buddhism know practically nothing about it. It's almost a meme at this point.
Anyway, as to why it's because they think it makes them look more "cultured" because buddhism is an "exotic" religion in the west. It's an aesthetic to them, nothing more.
It's sort of like how a lot of anime has christian symbolism. Evangelion would be the most obvious example but plenty of other anime have christian symbolism though it's usually more subtle. It always has either zero relevance to the plot, or it just makes no sense, but it's there because Japanese see christianity as "exotic".
Anyway it's the same concept, it's just an aesthetic....though it's annoying when white people do it because there's a layer of smugness attached, (again, they think it makes them look more "cultured") whereas Japanese people just think crosses look cool.