The cages are meant as a metaphor for:
* the feeling of dependence upon the conveniences of modern society
* the desire to transcend reliance on pollution-generating technology
* and the circular, catch-22, hypocritical trap this creates within the conscience.

When I was biking regularly I found these converters on the streets of Portland at a rate of about one per month. The catalytic converter is intended to reduce toxic emissions levels from the automobile. But apparently they fall off a lot. You might want to check your car to make sure you've got one.