French Farmer arrested for driving with bio-fuel

From The Independent:

"Olivier Lainé, 49, was arrested near his farm by French customs officers. He faces prosecution for driving a vehicle powered by an "unauthorised fuel" - namely pure vegetable oil, made from colza, or rape seed, grown on his own farm.

...

The use of vegetable oil as fuel is authorised for vehicles while operating on a farm. It is illegal to drive vegetable-powered vehicles on public roads because no tax has been paid on the fuel." (read full story here).

Why does the phrase "unauthorized fuel" make me uneasy?