This is quite an interesting story. I will mention why after addressing your questions.
Don't let them get to you. They have NO idea what your goals are. In terms of your back rounding, you really don't want it to round at all. It doesn't take much to cause injury. I know a guy who can bench press 400 lbs, but when he lifted a small couch, he torn his bicep clean off the bone....so you always want to be aware of your technique, as you are because you mentioned your back tends to do that. A good rule to have is once your technique breaks down, end the set right there. It's possible you may never get hurt, but especially with squats, dead lifts of any kind, and any other heavy duty exercises, you always want to have good posture throughout. Those are dangerous exercises even with perfect posture.
Anyhow, many guys do it because they feel as though weight lifting is a "guy thing" because traditionally before much was known about fitness, men lifted weights, and women went walking. So some guys still have that thought that males are superior in the knowledge of weight training...even if they have no formal training on the subject! Some guys may do it as an ice breaker. It provides them with an open door to talk to you. This is what has me hesitant about joining a gym here in Nashville, because I can't stand when people always want to put in their two sense. I am like what one of the previous persons replied with, about feeling as though you have the obligation to. Many people who have a thorough education in fitness feel as though they always have to say something, because fitness is their life.
Here's the interesting part. Most guys, especially the gym rats don't know what they are talking about. Their education may have only come from others in the gym, and muscle magazine. I'd say at least 60% of men, and I'm estimating, who perform squats, dead lifts, and power cleans should not even be performing those exercises, because they do not have the range of motion in the joints to perform the exercise properly.
When I'm in the situation, I just say, oh okay, thank you. That way they are satisfied you listened, and you can just continue doing what you do.