Considering that this is a statement made by a US agency, and the primary purpose of such an article is to convince the people that are defunding them, primarily the US taxpayer, to continue to fund them, the football field unit is completely appropriate. This is particularly true because most US high schools, and some middle schools have a permanent football field near them, and even parks have football fields temporarily set up from time to time. Anyone in the US who has tried to get an education, and even those who have slept during class, are likely to have walked by football fields enough to understand what the size of them is.
It is certainly more likely that someone in the US will have a feel for the size of 1/2 a football field than 50 yard sticks or 150 ruler lengths. There are very few other standard sized objects on that scale that people have real world experience with. The reality is that most in the US that understand 50 meters will approximate that to 50 yards, and then imagine that distance compared to a football field. NASA is just doing that conversion for those that don't have a feel for meters, and for those that don't realize that a football field is 100 yards.