Presumably you have a research proposal. Why don't you post it? It's not like it's going to retrospectively change what people post.
Also note that the Piston area exists in a limbo area - it's purposfully not scrapabke by google etc. Of course you could register an account to scrape it but that would seem like bad form to me.
What's your rationale for stating this? It is still a public forum, only you need to have a registered log in.
As an example, let's take one very good reason for scraping - analysing a user's posts in order to put together a case to prosecute 'hate speech' (and piston required to be accessed for this).
From section 2 of the Prohibition of Incitement To Hatred Act, 1989
2.—(1) It shall be an offence for a person—
(a) to publish or distribute written material,
(b) to use words, behave or display written material—
(i) in any place other than inside a private residence, or
(ii) inside a private residence so that the words, behaviour or material are heard or seen by persons outside the residence...
(c) to distribute, show or play a recording of visual images or sounds,
if the written material, words, behaviour, visual images or sounds, as the case may be, are threatening, abusive or insulting and are intended or, having regard to all the circumstances, are likely to stir up hatred.
Now, the law exists for a reason. I think it is extremely important that it applies online just as much as it applies in the physical domain. Perhaps even more so in view of certain tendencies stoked in the online domain... Just because the piston is only accessible through a login should not preclude it from being subject to normal law, perhaps analogous to above - "the words, behaviour or material are heard or seen by persons outside the residence...".
And there may be a multitude of other good and proper reasons for scraping a forum. The only concern I can think of, is if the process causes undue server load, likely to affect the normal operation of the site. But such consideration might easily be built into the software.