Violent video games...the problem is hardly the jelly-blood of COD, the bigger problem for lil kids would be the people they play with, when i play my PS3 i get on my mic, talk with my friends and we'll curse, this may or may not be a problem for 7 year olds. Many of the kids that play watch violent movies, the same thing it really doesn't make a difference in a kid unless its all he thinks about.
Research run by who? on people of what ages? and do they play all day? or do they play once a week? what is their backround? most research done like this is helpful, but only to one party of gamers, COD, for example, being the biggest game out their means there are tons and tons of different people types playing this game. You cannot judge the general public by one piece of information, not everyone responds the same way, most people i've met are not at all bothered by violence in video games, because they know it is just a game.
as to your other point, parents can allow or cannot allow children to play whatever they want its THEIR kids, it is a totally subjective matter.