What's under the union label? Sometimes it's a temp.

The image of the earnest union protester seeking redress from maltreatment, an image won often at a heroic price in years past, is being tarnished by the recent practice of using low-paid non-union temporary workers as union protesters.
This may have come to a peak in a recent Daily Show episode, in which a union protesting Wal-mart for paying just minimum wage with no benefits was challenged to explain why their paid protesters, temporary workers standing in 107 degree Nevada heat, were themselves getting minimum wage with no benefits.

Unions are hardly likely to advertise such use of temps, and yet a quick google of "union uses temporary workers as protesters" reveals at least one other recent case. In particular, the Washington Post reported another such case in July.

It makes me wonder how common a practice this is, and whether it is a new development.
Temporaries are commonly used to substitute for union workers in strike situations. But who knew that the very picketers themselves might be temps as well?