'easy field' simply represents a way of saying that there is no opposing players on the field, therefore it is easy as no-one will shoot back. I don't think that it has anyway of offending or demorilising players, if someone wanted to do that then they would simply say that they are rubbish (but with a different word). Someone could simply say to get the flag in game when the opposition is on the field, whereas easy field will not be shouted untill all of the opposition is eliminated.