At my college, its not particularly expected for non TT to be on committees, BUT since so many VAPs and long term adjunct positions get converted to TT positions, with the current holder often being a prime candidate, such people do like to serve to get the experience and give the appearance of being involved.
This is great for the adjuncts there. At some institutions, the unspoken rule is 'once an adjunct, always an adjunct.'
Untenured