There are no free lunches in life, Martin. There must be some catch hidden away here.
The airport taxes in the UK are quite huge, I believe airlines have to pay taxes per pax, but if it's a lump sum (per aircraft, per route), it might be profitable to give a free ticket and make an extra 20 pounds or so on airport taxes, handling fees etc. Anyone more info on this procedure?
Or maybe it's only valid if you take an expensive return trip or so, or maybe it's some sort of temporary promotional to gain attention on the brand or new routes.
Otherwise, they can better keep these seats empty as the farepaying pax will be happy to have some empty middle seats, and every marginal passenger cost a few $ on additional fuel to lift the 150-250 pounds of 'load' , handling time, paperwork etc.
And all the more, the marketing communication effect of permanent free tickets would be negative, as farepaying passengers would be annoyed by the knowledge that their neighbor got it's seat for free while they paid 100 pound or so.
nobody has ever died from hard work, but why take the risk?