I find that highly unusual compared to code requirements here in the U.S. Copper used as a natural gas conduit is not condoned for a couple of reasons. Firstly, the copper, when coupled with the service gas line coming to your house from the gas company will probably connect to a "black iron" pipe (again, here in the U.S.). This sets up a perfect example of electrolysis... i.e. corroison due to disimilar metals (Galvanic corrosion). The copper will corrode and eventually leak... more so if it's exposed to weather.
Secondly, copper pipe, when exposed to natural gas suffers a build up of Copper sulfate on the inside of the pipe. This will, over time, flake off and cause valves, burners and other orficies to fail.