Thinking of this from a business point of view, East Coast will know that loadings in 1st class were down due to the recession, as businesses look to cut back on expenses and travel all together. We all know that some TOCs make a good profit margin on 1st class customers, even if they only make up say 10% of total passengers transported.
So what do you do in this situation? You look at the other markets going - so leisure and day-trippers. If these type of passengers make up some of your other 90%, then even if you only entice a small percentage of these travellers to go 1st for a change (so hence the £25 and Reader Offer promos), then you get a few more into 1st. And you never know, some of these people may enjoy 1st so much that the next time they travel, they might think it is worth the extra few quid to go in 1st instead of standard.
The complimentary offer is designed again to do that. I'm sure EC have worked out the total cost of what a reasonable person is likely to eat and drink on a trip, whether it be above or below the 70 minute timescale. Ideally you'd have some people not taking up the offer, and some who will have more than average. It should balance itself out, but clearly on some services it doesn't. Perhaps the hesitation to give out more than a beer, soft drink and spirit is the fact that there may not be enough left for someone else getting on later in the trip. It may also be the case that those who travel for less than the 70 minutes are effectively subsidising those who travel a bit further, and therefore get a bit more.
Comparison with the airlines is tricky as it has already been pointed out that you know the maximum amount of people that will be on the plane, and you don't when on a train due to the different tickets. Plus, planes normally travel A-B and don't usually stop half way and allow more people on.
It would be quite foolish of EC to not have looked at the economic case - I've seen part of the manual given out to Subway staff (sandwich shop), and it shows how much of each ingredient should be put into the sandwich. It even shows which way the cheese slices should be arranged, to make it look as if the sandwich has more filling than it actually does.