I am personally of the opinion that people consider them religions because the people who call them that are usually people who do not participate in the lifestyles, and therefore consider them to be incorrect. In this way, it's easier to criticise them if they are addressed as "just another religion that's got it wrong", rather than having to actually examine what these lifestyles are all about and make a more detailed judgement on whether you agree with them or not.
It also has a lot to do with xenophobia, and ways in which these lifestyles contradict the beliefs of the person talking about them. Xenophobia, in that Eastern belief (in comparison to Western Christianity etc) must be inherently wrong because it does not adhere to the stories of the familiar Abrahamic religions. Because they do not match up entirely in -this- regard (i.e. what makes Abrahamic religions distinct from others), they must therefore also be a religion of sorts, just different. I have to say, the name "Satanism" really really doesn't help the Satanist cause. Much as I feel they're unfairly prejudiced against, it was a bit stupid on their part to name a belief system after such a controversial icon of evil and then attempt to go back and say "But that's not what we meant!".
Basically, I put it down largely to ignorance on the part of those who label them as such.