fema is an american govt agency that stands for Federal Emergency Management Agency.
Do they have access to walmart, yes...
Why? Because if an earthquake or disaster happens they will have ability to quickly control the distribution of food to help prevent looting, and to help prevent the food only going to 25-50% of the people it could have... This will increase the chances of survival for many of the residents.
Can they use this in time of Martial Law, or Foreign Attack?
Yes, that is not the biggest concern but it is one of the concerns they have taken into consideration when developing emergency management plans...
Be careful on youtube, a lot of videos take some truth and stretch it into a full lie... Correlation does not imply causation...
A youtube channel that is very popular that I would highly suggest to be careful of is Jason A... Others include Dahboo, Alex Jones, and basically 99% of the youtube channels that are truthers and come out with videos daily or multiple times a week.
Edit: In short, yes fema has tons of power in emergency situations... Because they are Emergency Management... These emergencies can include martial law, and foreign invasions: although the likely hood is very low... They take this into consideration and next thing you know, people are talking about 666, martial law, and rapture...