In some cases there may even be an advantage to coming from elsewhere (for example S got a UCDC scholarship to pay for summer room and board), though once you are off campus and paying rent in DC for 12 months then you may as well stay.
But more importantly $15K extra per year goes a long way and would more than cover an unpaid summer in DC. If your kid is limited to paid opportunities because they need to earn money in the summer then it makes everything much more difficult.
S was very fortunate that his think tank job paid $15/hour, but it was the exception rather than the rule, and limited to top students mostly from well known privates (Harvard, Yale, Chicago, MIT, etc) or state flagships.