play Rome for a few months and then cancel the pre-order for Empire (which also revokes the free game, you don't get to keep it if you cancel the pre-order).
That is precisely what I'm considering doing. I was going to read the details of the pre-order tonight when I'm at home.
And you're looking at this all wrong. They are offering this bonus as a way to get people interested in the new game. And in my case, it's worked. I'm not certain I'm going to buy the new game but I will be pre-ordering it to play Rome and see if I want to play more.
After watching the video clips on gametrailers.com I'm VERY impressed with the naval combat and the game in general. It looks like a more advanced version of civilization at some points. I've heard people really like this series but it never caught my attention enough to make me want to pay for one of the games. I'm a sucker for naval combat so assuming this game will run well on my machine they probably just sold me a copy by offering a free gift.
I would not have considered making the leap if they'd tied the Rome: Total War game to a purchase instead of a pre-order. Their intent was to get me to try Rome: Total War in hopes that it would encourage me to buy Empire: Total War and it's looking like they are successful.
It's possible that after playing Rome: Total War I may not like the game and may choose to cancel my pre-order. I may get to keep Rome, I may not. But their objective has been achieved. They got me interested in their game by giving me a way to sample it for free.
You've heard the expression you have to spend money to make money? This is a perfect example of that, they make an offer to potential customers and if done effectively there will be a percentage of people who become paying customers. Which is what you want. You won't ever get 100% conversion.
By your logic anyone who reads an advertisement, or looks at a promotional video for free and doesn't buy that game is abusing the system. Those ads, video clips cost money to create and put in front of you, and many people never pay for the product.
This is no different. They are putting Rome Total War in front of players in hopes they will buy the game. They are probably paying for this promotion in some fashion, or steam is, who knows.
It's all spending money to make money. This isn't a reward for people who know and love this game, it's a way to encourage people who's never played these games to get involved without losing money if they don't like it.