I believe I can shed some light, as I was the “perpetrator” of said offense. I am playing the Doggie Lord in ME76.
Yes, I can completely understand everyone’s concern over this matter.
Had a FP player informed my group that our messages had been wide open, “concern” would likely be the mildest of reactions.
I found the FP Yahoo group while looking for the DS Yahoo group. Is it my fault if a search using keywords of “MEPBM” and “76” brought up their Yahoo group as a result and not only that but the message section was wide open? (The DS Yahoo group as it happened was hidden, and on the UK Yahoo site anyway)
Now, obviously you have only my word that I did not read ANY messages. And you only have my word that I did not reveal this to my teamates until AFTER the problem was corrected.
So what DID I do? I immediately contacted the freeps, BEFORE turn 1 ran btw, and asked them to change the group to private so no one else could find it as I had. (Obviously a deceitful attempt to cover my espionage by appearing concerned, right?)
If I had really wanted to gain advantage in the game through cheating I would never have bloody told them in the first place. In short, I reasoned I could be silent and if someone else had found the group I would just sit back and say “not my problem”, or I could (and did) let the enemy know they were compromised. Yes, I figured people would suspect that I’m a liar and had read their plans, but I thought that not as bad as the possibility that we all pay and play, and then 10 turns down the road find out someone had been reading the freeps messages and cheating the whole time. Not a fun alternative I imagined. Lucklily this all occured prior to turn 1 so the FP could (and did apparently) contact Clint and voice their concerns.
It’s turn 3 now, so I imagine it was decided this was not a disaster of game halting proportions.
Now, if the DS end up winning I suppose every FP player will in the back of their mind always think of this as the game the DS cheated. Wonderful. Well, it seems likely to me that we will win, but not because of knowing any of our enemies moves beforehand, it will be becuase we will simply outplay I predict. 
btw, this is only my 3rd MEPBM game, but from what I’ve heard and seen by playing FP so far the DS don’t need any more of an advantage.
So, I ask you all. What would you have done in my shoes then?