It's never implied that they ever locked the cells systematically at any time they've been in the prison. So it shouldn't be a case of simply losing your guard after 30 days.
And I'm sorry, the setting of the show is post-zombie apocalypse - taking precatuions is common sense, and has been demonstrated consistently in the series up until now. I don't consider locking your cell as you sleep paranoid, just like I lock my car or house as I leave it.
We know people turn as they die, so if old man Hershel decides to kick the bucket (forget the flu), we'd have a zombie running around eating people at night. I've been willing to suspend my disbelief watching this show but having every prison cell unlocked swung open while you sleep is pretty stupid and a contrived plot point.
You're right (WHAT?! I know! A BSN member admitting to being wrong? xD) But I still choose to empathise with them. I could go and make a speculative assumption of these mistakes being confined to that one area, but it's a pretty silly leap to make given the information we're given.
I still feel that after months of this nightmare, people want to sleep without fear. Which is silly, of course, but people do stupid things like this all the time: it's hardly uncommon. One could even go to the point and argue its more human than any reasonable and practical manouver.
In this case, it's obvious that they needed the **** to hit the fan. Personally, I would have written it so that the entire area was locked and that they (black dude and white chick) left the door open foolishly. Now, how exactly would the **** hit the fan with the rest of the cells being locked? Simple - It wouldn't. There was no need for a zombie attack within the prison when there are so many opportunities for it to happen later on.
As for the writing, I think it's good. Not Breaking Bad good, but a solid 7/10 from me. Of course, you cannot compare it to literature since those dudes have all the time in the world and no visual medium to work with other than their imagination. If the writers didn't need to edit out their work so that it'd fit 45 minutes, I think it'd do a better job. TWD has its ups and downs when it comes to its writing: there are some amazing scenes like Rick and Shane's confrontation...and then there's Carol's "your weak" line. So far, the only season that's ever disappointed me was the second half of 3.