There's no good answer for this because a lot of stuff that, by the letter of the law, should get penalized is instead let go. It comes down to tradition or culture; it's just how hockey is played in front of the net.
A lot of this is a grey area where officials see "acceptable interference" vs interference that you would penalize. The rule of interference states that any player has the right to his own ice; i.e., the ice that he is standing on and reasonably occupies. Just because someone runs into you and falls over doesn't mean you interfered with them as long as you are maintaining your ice and not moving into their path intentionally. That's why you see a lot of players get in front of an opponent while skating and force them to slow down or change their direction. This is not interference because, again, the player is establishing body position.
56.1 ... A player is allowed the ice he is standing on
(body position) and is not required to move in order
to let an opponent proceed. A player may “block” the
path of an opponent provided he is in front of his
opponent and moving in the same direction.
When two players are in front of the net, they are often said to be "jockeying for position". Both players are essentially occupying the same ice, or attempting to establish body position in it. A referee will let pushing and bumping go and not consider it interference.
It in general becomes a penalty if someone gets crosschecked or tripped or slewfooted or otherwise forcefully knocked to the ice. It becomes interference if instead of general pushing and shoving, you restrain the player by grabbing and pulling with your arm or stick.
As I said, there's no good answer. Most refs turn a blind eye to most stuff happening in front of the net until it goes too far.