Why do a couple of formula 1 cars have a bar code on the rear wing?

May be...

The only 2 cars that have this now are the Ferraris. This is in place of the Marlboro advertisement. As cigarette sponsorship is banned in many countries, Marlboro (a main sponsor of Ferrari) has to take out their advertisement.

So instead of a blank spot or letting a someone else use the spot, they used a barcode. Not sure but believe if you scan the barcode, it would probably read Marlboro.

Tobacco advertising is banned in F1 so teams have crafty ways to get round it by manipulating names and images

Those cars you are refering to are the Ferrari cars. They are sponsored by Marlboro. Problem is Marlboro is a cigarette company & cigarette advertising is heavily restricted. So wherever it is banned they replace their logos with bar codes.
That's what you're seeing.reebok-umpire-plate-shoes