SwissIX is organized as a non profit association under Swiss law. (See our articles.pdf.)
A SwissIX participant does not necessarily have to be a member of the SwissIX association. This is mainly for legal and administrative purposes, as in some large companies it might become rather difficult to find someone entitled to sign the membership form legally.
On the other hand, some members of the association do not participate in the SwissIX peering infrastructure, because they do not operate a layer-3 IP network, i.e. colocation or layer-2 network providers. Still, those members support the neutral and non-profit platform SwissIX.