In the field of play an opponent scratches us with his nails, say, or gives us a butting blow with his head: but we do not 'mark' him for that, or take offence, or suspect him afterwards of deliberate attack. True, we do keep clear of him: but this is good-natured avoidance, not suspicion or treating him as an enemy. Something similar should be the case in the other areas of life too: we have people who are our 'opponents in the game', and we should overlook much of what they do. We can avoid them, as I say, without suspicion or enmity.
In the first passage, he is simply saying, "if it can be done by someone, then why not you too?" If it can be done and if it is "appropriate", then you can do it!
In the second passage, he advises that we ought not to consider "intent" so much. In a game of football, someone tackles you roughly. No matter - it is part of the game and you most likely think there was no mal-intent; it was just a rough tackle. In this same manner, we ought to approach dings and nicks in the sides of our car; a driver pulling out in front of us or someone cutting us off in traffic.
Would we let someone cut in front of us in a line at the store if we saw their child acting up? Would we let a young, 30-year old man, purchasing beer, cut in front of us? What if he did without your permission? When you step back and look at the situation, and simply observe: someone cut in front of you, no more no less. What you decide in your head as to that person's motive is entirely up to you. You can choose to be tolerant or offended.
Give others' the benefit of the doubt - it'll do your soul good.