I think it depends on the circumstances. I often would loosen the straps on the girls car seats when we were NOT in the car. If we were strolling around the store or whatever I would loosen the straps for comfort. Of course once we got back in the car everything would be tightened back down...and then rechecked after loading groceries...lol. (I'm a bit OCD.)
I'm sure many other Moms and Dads do the exact same thing as I did. Now if you saw them loading baby in the car and NOT adjusting the straps that would be a whole other story.
I do know exactly what you mean about wanting to say something but unsure how it would be received. One of my biggest pet peeves is seeing the baby seat propped up on the "seat" portion of the grocery cart. Especially when it's sideways and lopsided. One good bump of the cart and that baby is going down. Those infant seats are NOT meant to sit like that, there is no way to strap the seat to the cart. If your seat does not fit over the cart seat properly for goodness sake do not turn it sideways while it's leaning like the tower of Pisa and think your child will safe. I have seen several seats topple off of the cart and baby goes face down on the store floor and even saw one face plant in the parking lot as Mom went over the speed bump.
We usually shop together so our baby was in the stroller while the other one pushed the cart. On the rare occasion where I'd have to run out alone I would put her in the grocery portion of the cart and the groceries went in the seat portion.
Another common grocery cart mishap, allowing child to ride on the bottom of the cart. I've actually seen kids lying down on that bottom part where you'd put a case of water or bulking items. I've seen 2 kids get their hair caught in the cart wheel and 1 child nearly lost his finger in the wheel, literally was hanging on by a thread. I've seen children riding on the ends of the cart only to have their feet slip and bust their face on the cart.
I have tried to politely share the baby seat info with other shoppers when I see it by striking up a conversation about their adorable baby, how old is this little cutie, ect. and then saying something like "I had no idea how dangerous it was for the baby seat to be in this position until I saw a seat topple out onto the floor while shopping. It scared the bejesus out of me and I thought to myself "Wow they should put a warning on here for us unsuspecting parents." By using the word "us" I've included myself into the group of unsuspecting parents so as not to come off as a know-it-all but more of a concerned parent sharing "new" information. Most of the time it is received well especially by Dads. Other times not so much...but alas I do try.
Peace and Blessings,
T. B