Because if I were him, and my parents gave me that much money, didn't work, had a girlfriend I would at least save some of the money to take her out on a date. This is how I feel. But he is a kid, putting himself first.
Well both my parents and my now DF's parents paid for our cars, insurance, gas, etc and we did not have jobs in high school. To both of our parents, school was more important and they felt that having a job during the school year would interfere with our school work. They did, however, demand good grades. It wasn't like they were throwing money at us -- they just thought it was their responsibility to pay for what we needed (and we did need a car for several reasons which I won't go into detail) and it was our responsibility to do well in school. I am well aware that this is not how everyone parents, but this was our parents' philosophy.
DF did always pay for everything. But, should this be expected? I think it really just depends. Were they on a real date or just going out as friends? Did the guy think they were on a real date? You might be surprised how oblivious some teenage boys can be.
Although I will say this -- we had this semi-high school reunion about a year ago. DF wasn't in town, but two of my really good friends from high school were (both guys). Now, we're going to a high school football game and they know that we're going to have to pay to get in. Well, we get up to the gates and guess what -- neither one has any cash and guess who pays for them to get in -- me. They said they would pay me back, but I didn't really expect it back and told them not to worry about it. Now, these guys are not moochers -- they were just oblivious. They also aren't stupid either-- one is in grad school at Stanford!
I agree with the previous poster -- pick your battles. Your DD now knows that it might be kind of weird if he doesn't pay and she has that in the back of her mind; but now that the thought is in her head, I would leave it at that and let her decide what she is comfortable with.