Some people do it in England, it's not exactly unheard of. Friend of mine literally gets a "doggy bag" to take food back for her dog. But there is a sort of shame attached to it. You basically don't want everyone to know you are taking food home. Looks like you can't afford your own food or something (even though you are in a restaurant). Stupid I know, but is a cultural thing that has evolved I suppose. I actually know people who declare quite loudly to the others on their table (and so everyone in the room) that they want a doggy bag to take home to their pets. It's all ok then...
Heh, talking about restaurants, I personally just cannot complain about anything. Mainly because I get worried about what they will do to your food afterwards, especially if you want to go back. My dad will yell and shout, demanding a replacement, if he thinks the food is bad, but I never risk it. What you don't know won't harm you and all that, but I have a good imagination...
Cheers