Why people do and don’t join SPI?

I just asked this question over on another site and thought I’d widen it to here:-

I’m standing for election to the board of Software in the Public Interest this month, so I was wondering how many users who contribute to free and open source software have joined? If so, why? If not, what’s stopping you?

http://www.spi-inc.org is the organisation’s website. Even if I don’t get elected, I’m interested to know why people do and don’t join and would like to summarise the answers to SPI, so please mention if that’s not OK with you.

I’ll summarise some of the other answers I’ve had in a day or two.