Why are some lollipop notifications able to be swiped away on lockscreen? I have a pin set and privacy set to don't show sensitive content.

Some notifications such as tune in radio telling me about a recommended radio are able to be swiped away from the lockscreen where as some can't such as sky go notifications, texts email etc.

Why is this?
There are two types of notifications: Persistent notifications and swipeable notifications. Persistent notifications won't be swipeable because they are set like that by the app and for good reason (For eg: alarm or Poweramp or system notifications). There may be apps which abuse this. The only way to remove them would be to force close the app. And like you said, swipeable notifications can just be swiped away because it is assumed that they are unimportant.