Here's something to think about. There are kids at USNA who already have (or have access to) more money than most of us will make in our lifetimes and also those who are dirt poor. There are more than a few mids (quite more than a few) who not only get zero money from home, but they scrimp and save and send their money home to their families. The fact is that any mid can not only survive but thrive at USNA with the money he/she earns. And sponsors help by providing a get-away (meals, a different place to sleep, entertainment) that is free of charge.
Obviously, if the kid wants to do something like NOLS, which is very $$$, or a sport/ECA trip that USNA isn't underwriting, the parents are likely going to have to help out. Some parents help their kids buy nicer class rings or help them buy a nice(r) car. Most send care packages and most pay for flights home. And so on. The above is totally understandable and great for those parents who can do it.
The above said, I personally (and this is only my opinion) find it somewhere between shocking and horrifying to think that parents are giving their mids a monthly stipend. It's not a question of whether they've earned it or deserved it. It's the difference between a mid (or cadet) being a paid member of the Armed Forces and being a normal college student, where the kid/parents have to take out huge loans, etc. These 18-year-old members of the military are not children. Continuing to treat them as such won't help them become adults. IMHO, giving a monthly allowance/stipend to a mid/cadet is simply a means of trying to maintain control at a time when there really should be no more parental control.
[As an aside, every parent/family has the absolute right to do whatever they want. And I've no doubt that there are many mids who get a monthly allowance from mommy and/or daddy for 4 years and go on to greatness. But, based on experience, I think this is a great time to let your child work things out for him/herself and yet be there (if you're financially able) to help out at those times when things don't go as planned or to give you child an experience at USNA that he/she couldn't o/w afford. Just my $0.02.]