Biofuel is something of a developing cottage industry with many players and spotty regulations and standards. The fuel has more variability and associated risks regarding its source material, production, storage and distribution compared to standard diesel.
I think the established fuel industry, such as Exxon Mobil, have some liability from having their name on stations that dispense a fuel mixture that is relatively problematic and over which they have relatively less control.
The sign is an indication of this situation.