Mom forbids 18-year-old from spending $30 of the $400 he earned from winning a competition: 'My mom said that since she paid for my travel expenses, it's not my money.'

He earned that money, and he should get to do with it what he pleases. 

Parents don't like sitting idly by while their kids make stupid financial decisions. If your 16-year-old son spends his first-ever paycheck from working night shifts at the Dairy Queen on fast food for him and his buddies, you're likely to have this instinct to shake him and yell, "WHAT IS WRONG WITH YOU?" But when your kid is making their own money, you can't always control how they spend it. You can do your best to instill good financial habits in your children by not bailing them out every time they spend all of their money on something stupid. You can ground them for going against the rules of the household if you instill a rule like, "You must save at least 10% of every paycheck." Heck, before they're 18, you, as their parent, can technically use their money however you please. I wouldn't advise doing the third option, but it might be necessary if your kid is actually financially out of control. 

While some parents feel they should intervene in their kids' financial lives because they're not mature enough to take care of themselves, other parents just want to control their kids, even when they've proven themselves financially stable. As far as I see it, as long as a kid isn't being dangerously irresponsible with money, their parents should let them learn from their mistakes and make their own choices. They should be in charge of their own money, especially if they're 18 and earned it themselves.