The answer to this question is yes... and they are right to make that assumption.
No I am not saying you are an idiot but it is a sign of a competent DM that they take the time to check divers gear. There are many more stories of divers going over the side without checking their gear and having a failure than there are of DMs screwing up divers gear.
I appreciate that you are careful but a dive operator cannot and should not make that assumption unless they are familiar with your habits.
And I am right to make the assumption that the DMs are incompetent, bored, and likely to make errors. Most are not and do not, but then neither do most paying customers fail to open their own valve.
I think the excessive double checking of each customer's equipment has made many vacation only divers, especially the newer crop, into completely non-self reliant divers who actually expect the DMs to set all their equipment up for them, hold their hand during the dive, and keep track of their air consumption. This is what I had in mind when I wrote "when I look around me".
The DM may have good reasons to check my equipment because (s)he does not know me, But since I don't know her or him I make sure I double check anything they do that concerns my equipment. I will stop at the boats edge and recheck my valve and anything else that was touched by anyone other than me. I make the assumption that I'm smarter, more competent, and have better diving skills than they do, and I'm generally correct. I am polite unless they get surly. It cuts both ways.