To the OP for the original question... yes its very normal, esp with maroon clowns they are very territorial.
As far as the relationship between an anemone and clown. Well, there is a ton of speculation and no real factual conclusions, (yet). There are theories that state a clown adapts its slime coat to accept an anemones stings so that they do not bother them. However there are other theories that state a clownfishes slime coat acts more like a numbing agent to an anemone, and the it cannot detect that there is anything there. I am leaning toward the latter of the 2 personally but thats just MO. If their slime coat were to adapt to accept and neutralize an anemones stings it wouldnt stop the anemone from stinging or trying to grab the fish every time it was there. However, IME, everytime my clowns go into my anemone it reacts no differently than if nothing was in there. However throw a piece of food in and its a completely different story. I also do not believe that any clownfish feed their anemones on purpose. I believe they try to store their food in their home. The anemone is just lucky to have such responsible house guests.