John,
Model Boats stopped the givaways years ago as it was prohibitively expensive. They had to pay for twice the shelf space in newsagents because of the thickness and of course subscriber postage went up too. It made no commercial sense as only a tiny fraction of the readership were ever likely to take up whatever offer was going, usually a separate plan. That is why the free plans these days are stitched in. It's much cheaper to print them that way.
Starter modellers are encouraged by straightforward designs such as those by Glynn Guest and also by the wood packs advertised in the magazine which people can purchase if they wish. Model Boats and similar niche interest magazines are run on a shoestring budget, there is no spare cash to take a punt on a freebie which only a tiny proportion of readers might actually want. It is more important to try and keep a lid on the issue price to keep the readership you have already got!
With a specialist publications such as Build the Bismarck, only those with an initial interest in building the model will buy it. OK the Bismacrk has been done to death but the publishers need to have a subject which people are likely to have heard of so we get the Bismarck on th naval side and the ever popular Titanic on the merchant one. To be honest I doubt if many people stay the course with these publications and no doubt that is factored into the business case. Also, as someone has already said, a completed model will never be worth the cost of all the issues needed to make it but that is pretty much par for the course in model boating generally where kits rarely command more than the original price plus some of the running gear etc. But of course you shouldn't embark upon a project like this if you want to make money – it's just a hobby.
Colin