The first thing that comes to mind is a religious mandate to eat meat in a theocracy (or a government with a strong religious foundation, even if it isn't technically a theocracy). Why the religion would demand that people eat meat is up to you. Perhaps certain animals are thought sacred, and souls are strengthened by members of the flock eating them regularly.
Alternately, maybe there just isn't a lot of produce available in the area and it needs to be rationed. Since vegetarians have to eat a lot to get the calories they need to survive, vegetarianism is outlawed to stop them hogging all the vegetables. It doesn't have to be an outright ban either; if vegetables are rationed, it might simply be impossible to live as a vegetarian.
A public health argument is possible. If the health care system is government run and they believe vegetarians cost more money to care for than people who eat meat, it's completely believable they might outlaw vegetarianism or discourage it with taxes or fines.
Perhaps there was a very active eco-terrorist movement of radical vegetarians in the past that caused lots of problems, and vegetarianism was outlawed as part of the government response. Vegetarians are now looked on with extreme distrust and assumed to be the spiritual descendants of the eco-terrorists of the past.
It really just depends on the social context of your fictional country. Many of the laws we live under exist not because the things they prohibit are bad in and of themselves, but because society has decided they shouldn't be allowed for whatever reason. Given that, if you lay the right groundwork practically any justification for outlawing vegetarianism will do.