I'm not sure if this is a cultural difference between France and the states or if I'm overly paranoid, but if I were to receive a summons of any type by any government agency with more authority than the dept. of motor vehicles, there is precisely nothing that would get me to comply with that summons without talking to my lawyer first.
Speaking for myself; if MI5 (to pick an example) contacted me as a public and relatively high profile UK editor/sysop and asked me to come in and, say, advise them I probably would.
I keep forgetting we are reading a lot of translated text here; so "summons" might not be the right word in this context. I perhaps jumped to conclusions given the stuff that happened once he'd attended the meeting :)
The OP writes that he was threatened to be held in custody (French: "garde à vue"). This is a common trick of the French police: when people are held in custody, they have the right to an attorney and to keep silent, which the police don't like. So the police question "free" people and threaten them to hold them in custody if they don't cooperate obediently. Most people (especially good citizens) are scared of being held in custody, so the trick works quite well: people answer every single question and have no attorney, despite their right (and interest) to do otherwise.
In the present case, the charges (had any been pressed — he was there as a free person) would have been akin to terrorism, which means that the custody would have been under anti-terrorism rules: a very unpleasant experience that can last up to 96 hours. So the pressure was very high, despite his being "free".
ErrantX also brings up a good point about translation, "summon" could mean something as simple as a phonecall asking for a meeting, without any implication that one may be in trouble. Probably not something one would expect to find themselves being threatened with jailtime.
I doubt you'd need to worry about receiving a summons.
It's not what very many at all in the U.S. do though -- even those who think in the abstract that they wouldn't talk to the authorities without taking to a lawyer first (or at all) -- end up doing so. In part because the authorities are pretty expert at getting you to talk to them (by intimidation or persuasion).
But the more you think about it in advance and prepare, the more likely you can remember to exersize your rights.
I can't say for France, but in the U.S., remember NOT, ever, to talk to the authorities without talking to a lawyer first. It's really hardly ever going to make things better for you or your friends and loved ones. Don't talk to the cops.
How could I possibly justify not contacting a lawyer? I'm not qualified to read such a letter in the first place!
Edit: I see that you are asking about telling counsel about the NSL. The text of the NSL explicitly states that you may seek legal counsel.
Pressure works differently on different people and those in alphabet agencies know what buttons to press.
....maybe they'll leave me alone, if I just talk to them. No need for lawyers (OMG I don't even have the rent money this month!) and no uncertainties over life in prison. http://www.youtube.com/watch?v=6wXkI4t7nuc
(WMFR has no control over Wikipedia content or servers.)
Fixed that for the GP.
If the chapter, as an entity, tried to "interfere" in one of the Wikis it would go down very badly within the community. There is a certain amount of push and shove that means chapters are extremely careful to be seen only ever as facilitating.
But that's a whole rabbit hole of politics beyond the scope of this discussion :D
From the article (thanks for linking it btw, it has slightly more info than the English text in the OP):
"This volunteer had no link with that article, having never edited it and not even knowing of its existence before entering the DCRI offices."
So I'm guessing he complied with the summons because he didn't know what it was about and therefore saw no reason not to.
There's some comments below the article you linked, criticizing the sysop for not refusing to delete the article, calling it an abuse of his powers. Well. I don't know who this sysop is, but if he's just an average Jacques, never having had to deal with strong-arming by intelligence agencies, I can totally understand why he chose that course of action. After all, you can always undelete an article, and right then, at that moment, things must have seemed pretty scary to him. From the (English part of the) OP, it states that he tried to talk them out of it, that despite his role as a sysop it was not his decision to make, point them to the Wikimedia Foundation, etc. Who knows what they threatened him with (probably jailtime), I can't blame him for not risking that, or at least not risking it until he gets a clearer picture.
He probably thought, let's get myself out of this first, and let the inevitable publicity shitstorm sort things out later. And indeed, apparently the article is back up.