They do inhibit bowel activity somewhat, but I think the 7-day claim is pushing it - something else must have been going on... I think at my worst stoppage I've gone maybe 2-1/2 days or so. They don't want you to have to shit in the middle of a mission, but they don't want you to have a fucking rock the size of Gibralter sitting in your colon, either. I've never actually had a problem dropping a duece when I needed to eating them for extended periods, but I've also rarely had to while in the middle of something that just can't be interrupted.
Assuming you stay well hydrated you can live off of them for quite a long while - far more than 30 days. That is given for an active lifestyle, though... Eat them while sitting on the couch all day and that will be one hell of an unhealthy diet. They are nutritionally adequate, even more balanced than most of the crap that most people eat if the entire meal is eaten, though they are very high on the carb side (for obvious reasons - high energy requirements), as well as calories (roughly 1200 calories, same reason). If you really enjoy them and don't get bored with them (or sick of eating the same menus over and over), and provided you stay hydrated, then I see no reason why you couldn't eat them pretty much indefinitely. Although you might want to find a way to add some greens to the diet...
Personally I love MREs - not all of them, but a decent chunk of the menus I can eat for a long time and not get tired of it. But I am not basing my long term food plans off of them for one main reason: they are relatively expensive.