a) You're making an assumption. I personally wouldn't have had a field day, I can't speak for everyone else on r/the_donald. Feel free to assume what you would like, but assumptions aren't fact.
It's an informed assumption. I saw how that subreddit reacted to i.e. Michelle Fields, and though that wasn't a comparable incident aside from the specific use of the word "assault", it's pretty clear that the subreddit is and would be willing to ridicule anybody for using that word over a minor incident (i.e. being hit with a tomato).
c) It is assault. http://www.press-citizen.com/story/news/crime-and-courts/2016/01/27/man-accused-throwing-tomatoes-trump-arrested/79399992/ I'm not downplaying it. He said- if someone tries to throw a tomato at me then knock the crap out of them. And someone did. And he didn't get knocked out. He got promptly arrested. People in the audience knew how to handle the situation when Trump said it, apparently.
I'm a Trump supporter. I know what Trump said and why he said it and how he said it, and that's why it doesn't waver me. His intent wasn't to get the people at his rally to be hostile towards others, and people at his rallies didn't feel like he was asking them to be hostile to others. Again, I can't speak for the whole group- I'm sure there might have been a handful of idiots who did think it was grounds to just attack, like that guy who suckerpunched a protestor being escorted out. But that's on their own basis of thinking, not Trump.
That's great, but I believe – and correct me if I'm wrong – but I believe the OP asked this question as a follow-up to this idea, which was on the front page of /r/the_donald earlier today: https://www.reddit.com/r/The_Donald/comments/4mcgf3/bernie_called_for_a_revolution_and_last_night/
So it's fine if you don't believe that Trump was intentionally trying to incite violence (and I'm inclined to agree with you) but there's a ridiculous double-standard, especially when the rhetoric out of that sub has only grown more absurd and heated (calling violent protestors "terrorists" and "rioters" with a seemingly dim - though probably affectedly so – understanding of what those terms invoke).