Although there is an argument to be made that just because someone is in receipt of food stamps they should not be denied the occasional luxury, I believe the larger issue is that a vast majority of people not on food stamps work very hard for those same luxuries. Especially in the case of junk food this is an opportunity to provide an incentive to get off the system and provide for themselves.
Food stamps are supposed to be help from the government for the less fortunate, not for people who want to eat fancy or eat junk. This benefit is mostly awarded to people with kids and such, why would kids need to be eating junk, lobster or junk foot? The United states has a serious nutrition problem, maybe the government could steer these ignorant people into making healthy choices.
The government has more important things to do than harass the most disenfranchised of our society. The last time I checked steak, lobster and junk food were all food. If my understanding is correct, most don't even get enough food stamps to stretch for the entire month so I'm sure it is only a very select few who are able to buy expensive things.
Food stamps are able to help many people afford food, when it would otherwise be hard. However, just like someone without food stamps would save up money for a fancy meal like lobster, those with food stamps should be able to do the same. If they want to forsake a few meals that week to have a steak, they should be able to make that choice. Food stamps are a limited amount of money anyway, and you should never tell someone how to use a gift.