Idol trolls are win.

There is nothing more entertaining than an idol trolling someone else, whether it be a fellow member, another celebrity on a variety show, or even the fans.
Rena having gained this much confidence from her rise to supreme SKE frontgirl status is actually heartwarming to me. Sassy Rena is just so much fun. I mean, the sheer brilliance of a popular frontgirl writing a "don't comment on this" post is just cake to someone like me. At this rate she'll be reaching Shinoda levels of trolling, and that day will be AWESOME.
It's actually even more heartwarming to see that most ended up obeying her request, because I can tell you that a Western celebrity doing the same thing wouldn't yield any sort of the same results at all. This kind of thing shows why Rena deserves a spot in G+ senbatsu, because there's two-way interaction, not just a one-way idol ---> fan IV drip of management sanctioned fluff.
As for idol preachiness in general, that rarely bothers me because it's so easy to ignore.

If an idol's blog or feed becomes too preachy, I'll simply not read their entries and just enjoy their non-preachy existence in other media. So no, I don't really care if Rena's being too preachy or not.
The other thing being that similarly to what I said above, an idol speaking out like this is a rare instance when we can see an idol outside of the censorship wall of management. It's one of the few times we can know what an idol is ACTUALLY thinking, whether we agree with their opinion or not. For that reason, I'd gladly accept more preachiness, simply because it allows me to know her better, and know that it is her that I'm getting to know, not an image designed to make bank.
That's why Mayuyu's 2010 sousenkyo bitch fit was far more interesting and appealing to me than her more prudent but BORING "smile and nod" speech the next year. For a few glorious minutes we got to see the real living girl beneath the cyborg exterior. I certainly do admire her savvy polish and professionalism, but having at least the illusion that we also know a bit of the "real" her is what separates an idol from a normal celebrity or actress.
I believe that Japanese variety is so obsessed with ridiculous games, especially punishment games, partially because it both forces idols to real show their true selves in moments of shock, surprise, fear, etc., and that how an idol deals with humiliation or bizarreness is more telling about their true personalities than any scripted interview ever could.
Compare to K-variety, which is hell-bent on placing their idols on pedestals and praising them to high heaven as "the ideal [insert job here]" and while I love my Kpop idols too, (surprise surprise, usually the ones who "break their image") I don't feel as much of an emotional connection to them. My Kpop fandom is more based in removed admiration for their abilities and skills.
But since many Jpop idols don't have those skillsets to use, my Jpop fandom is more based in personal admiration for how they've grown in their situations, which relies on the behind-the-scenes stuff, especially their opinions on various issues, from which we can frame how we interpret their actions.