If an insidious forum-moderator wanted to steer all his members away from home-grown and into feed-store-bought produce, he could do worse than run the 'grass causes sour crop' line.
Not only do the chicken-health-conscious have to avoid feeding grass clippings to birds, they have to avoid putting it as litter in the pens.
There may be a case for the 'grass is dangerous' claim, but you'll notice that people who claim it very often have a high interest in advocating antibiotics for illnesses, and in shushing anyone who says they've used it safely for years by saying 'that's purely anecdotal' — as it happens, so is the argument that grass causes crop impaction (instead of saying it may be faulty crop/intestinal flora combined with fibrous feeds).
I'm not going to say grass clippings are good pen material because they're not — they cake together like pancakes and you can slip on them when the ground underneath is muddy. But when you have nothing else and especially if you can mix other materials (like leaves) through, you can stop the pancake effect and help reduce the amount of manure building up on the surface of the ground.
As for feeding lawn clippings, I'd tend not to only because of the mower gases and oils that would have spread through the grass. Certainly birds starved for green pick may gorge on the lawnmower clippings and make themselves ill. But I can see little harm in having piles of clippings everywhere if the birds are also given fresh green feed and have a variety of foods to help them develop stronger digestive systems.
I think I've written on this subject before, so forgive me if I sound like a harpy. But I do see the authoritative voice coming up often and I also see new chicken owners frightened into spending a lot more at the feed store than they may need to. The fact is, there is no better way to get natural vitamins into a bird than feed something fresh, whether it's greens or protein. So do be careful about using clippings, but don't avoid grass altogether, and if you're uncertain about safety, do what I do, and cut it with scissors!