You're a jerk, because you're trying to defend someone who probably behaves worse than you, by equating yourself with that person.
If they had permission and were okay with spending all that extra money on leftovers, it's between them and the restaurant. I don't like wasting food on principle, but now we're getting into fuzzy issues around the relationship between supply and demand. At the end of the day, if someone wants to spend their money on food and throw it in the garbage/compost, that's essentially their business.
I'd have more of a problem with it if it were meat, because there's no reasoning around the fact that an animal was killed for it. Rice is in more abundant supply with with less severe negative externalities to its production. We also don't know how much rice it was; "eight pounds" was an eyeball guesstimate that could be completely wrong or exaggerated for narrative effect.
And it's not character assassination when it's such a vague anonymous anecdote.
Your wording strongly suggests that you took it personally and were defending yourself and them together. If that's not what happened you need to work on how you phrase these things.
The top response at the time I commented implied that the individuals in question were "deliberate food wasters" and "despicable".
The reaction is mostly to your second comment, not the first one.
> The top response at the time I commented implied that the individuals in question were "deliberate food wasters" and "despicable".
Okay? It's still a vague anonymous anecdote.