Many people consider both Buddhism and Theosophy philosophies but not religions

 Even the Saint Germain Foundation, sometimes called "The 'I AM' Activity people call themselves "students" and often don't think of it as a religion but rather as a way of life instead.

So, it is possible that many people see Buddhism (because Christians believe Buddhist didn't believe in God) "which isn't always true" so they classify Buddhism as a philosophy.

Also, if you really study Buddhism it isn't really that Guatama Buddha did not believe in God what he really said was: "Arguments about whether God exists or not are not useful. We need to reduce the suffering of mankind and that should be our focus instead." Of course this is paraphrased because he didn't speak English because likely it didn't exist (at least in this form when he was alive anyway).

Also, religions often want to define a religion as either Christian or Islamic or Hindu or whatever. However, often professors of Comparative religion don't see Buddhism or Theosophy or Hatha Yoga or Accupuncture or things like this as religions but rather more as  medical philosophies to maintain good mental and physical health instead. Also, debate on almost anything is a serious part of Buddhism, especially the way the Dalai Lama presently looks at it and lives.

And like I said: "People in the "I AM" that I knew growing up often were insulted if you called "The 'I AM' " a religion too because they wanted it to be something scientific like a philosophy that people were trying to live by instead of a religion. So, the very idea of a religion was something alien to people in the "I AM". So, they would say things like: "I'm done with religion. That's why I came into the "I AM Activity."