The straw is an option that many people will request if it isn't given, so the server puts the straw there automatically out of habit to save time. You don't have to use it and the server knows that. And they don't care about the waste or cost of the straw. It's like unwrapping the napkin to find a fork and steak knife, even if you just ordered soup. The fork & knife will go back thru the dishwasher even if you don't use them. <I hope>
I prefer a straw because:
You don't know who or how the glass they give you to drink from was handled, or how it was carried around in the restaurant before it was brought to you. Some servers or other restaurant workers will carry several drinking glasses by pinching the rim between 2 fingers, or set the clean glasses upside down onto countertops that haven't been wiped down in 4 days. The people carrying the drinking glasses around the restaurant, like from the dishwasher to the glasses stack spot, or the server that brings it to you, do that chore just after they cleared a dirty table and picked up all the dirty plates, forks, nasty napkins and other stuff off that table with their bare fingers. Or they fix their hair that is falling into their eyes, or pull up the back of their skirt or pants, or reach down and tie their shoelace, the same shoelace that has been dragged across the bathroom floor on their last break. Then they fondle and caress your glass from top to bottom, put it in front of you so you can put your lips on it.
I love to eat out but have seen many servers that put their fingers in glasses so they can carry several at one time. So a straw is a comfort thing. I try not to think about those same server's fingers that touch my plate or put the rolls into a basket for the table. Or the fact that the liquid in my glass is touching the rim area that some stranger's fingers touched earlier. But I'm not a germ-freak, really!!
I really identified with the Jack Nicholson character (Melvin Udall) in the movie "As Good as It Gets".