As much as I generally despise the phrase "cheap plastic", there are times where being a low cost material can produce some unexpected advantages. Such as when people steal manhole covers in order to sell off the metal to a recycler. Plastic manhole covers bring less per kilogram for anyone recycling them, so the temptation will be less, even though it should be a lot easier for someone to steal them due to their low weight.
This is not the first time that plastics have been substituted to foil metal thieves. I wrote a couple of years ago about some local thieves who stole the copper piping used to cool a local outdoor skating rink. The Park Board replaced it with crosslinked polyethylene tubing.
Don't you dare think however, that plastic is so low in value to never be stolen. It is stolen frequently (1, 2 and 3). It's just that to make it worthwhile, you need to steal large quantities - truckloads and such.