Setting the record straight
I'm not surprised at the self righteous prognostication on this subject but decided to do some proper research.
The offence would not be theft but fraud, in terms of the Fraud act of 2006, this of course depends on which airline the 'offence' occured and in which country. Obtaining services by fraudelent representation . i.e. presenting oneself as a premium class passenger when knowingly not. The test applied here in the UK would that established in R vs Ghosh. That a reasonable person ( the one of the clapham omnibus) would know what he was doing was wrong.
However, looking for examples of cases and prosecutions of individuals in such cases I can find none. Surely, there will be some but they are certainly not as well recorded as prosecutions for other offences on board or against airline employees for theft and fraud.
I wondered why this might be.
First, that it would be reasonable for CC to check that passengers are sitting where they should be and challenge them if not. If CC knows that someone is sitting in the worng seat and then serves them a premium meal as a premium passenger knowing that they are cattle class then the CC is also committing an offence. Two wrongs don't make a right and if the airline wanted to make a complaint of fraud then both may be liable to prosecution.
Second. Even where offences have been perpetrated and the airline may wish to prosecute it is probably not cost effective to do so given that their loss is nominal. It is not something that happens very often, the cost is insignificant and the time and resource to follow it up isn't worth it ( CC appearing in court etc. )
Third, and this is speculation on my part. Were a prosecution to arise the ailine would be called upon the state their loss. Now many will argue that this is simply the difference between the price of a premium class ticket and economy class but it is not. The actual loss is negligible as the seat which was occupied fraudulently was only done so after the opportunity to sell that seat at the full price had passed. The prosecutors would also need to demonstrate that the fraudster had boarded the flight with the specific intention of sitting in Premium class without the appropriate ticket - very difficult to do.
Yes it is wrong. Yes it may be a criminal matter although there are civil remedies available. But, based on the evidence in the UK at least, it doesn't appear to be a problem that the airlines complain about or , if they do, that the authorities successfully prosecute.
Let's see which airline wants to draw attention the sparse difference between the basic service and premium service on short haul flights but the huge price differential!