See, if my parents said to me that they'd reduce financial support if I "had a larger entertainment budget" or bought expensive items, that would simply encourage me to stop earning my own money. Why earn extra for the luxuries in life if your basics are going to be reduced as a result?
My parents simply told me: "This is what you need to pay each month: $80 in car insurance, $30 for your cell phone and you need to put $200 a month toward college." Now, if I'd wanted I could have not taken any extra shifts and made my $350 a month which would have simply left me with $40 a month for lunches. If my parents were going to start demanding rent for me if I worked any more hours to earn more money, then I wouldn't have done it? Why would I work harder for no benefit? But instead, I took on more hours so that most of the time I was earning $500 a month and in the vacations I was earning $2,000 a month. MOST of this went towards college, but I also got a few nice purchases such as my shoes, the occasional CD, a vacation and good nights out.
It's like your boss saying to you, "If you do overtime, I'm reducing your hourly pay because we can only afford to pay you $X per week." Why would anyone do overtime for no reason?
Having the opportunity to earn more meant a) I was used to dealing with large sums of money before college and b) I got more work experience. When I went to college, my parents helped me out to the extent they could afford, which covered the basics beyond my student loan. If I wanted to go out partying, I had to earn that money. If I wanted to go away at spring break, I had to earn that money. New clothes, books etc. same deal. I DID go out and earn money, and the amount they gave me never changed. But if they'd said to me "if you earn money, we'll take away what we currently give you", why would I haul my butt into town at 7am 3 days a week to work an 8-hour shift in a shop for $10 an hour?
Now it's different if you're in a financial situation where you can only afford to give Johnny $100 a month at college: then he HAS to go and earn money to pay for the basics. But my parents said to me, "Kath2003, we are going to give you enough to get by. Not to go partying, but enough to put food on the table." The difference between my situation and yours is that you seem to be adding, "But if you choose to earn more money, we'll take the existing support away".
I guess I trusted that my parents were financially responsible enough to give me only what they could afford. If that had been nothing (and it has been at times), then so be it. It just means I work harder to continue with the lifestyle I enjoy.