Why are some lollipop notifications able to be swiped away on lockscreen?
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.