I don't think you are understanding the mechanism here. This is not costing $15B because they are consuming $15B worth of health care.
Medicare calculates what it costs to cover an average beneficiary by region, age, etc, and then private insurance companies can sell Medicare Advantage (MA) plans to people in place of standard Medicare coverage, and Medicare reimburses them for providing that coverage. Medicare is essentially saying "it costs $10K/year to provide standard Medicare benefits, if you guys think you can do it cheaper we'll give you the $10K as long as you provide the exact same coverage".
What are they reimbursed is also risk-adjusted so that if any given plan covers a sicker than average population, they get reimbursed more.
What is happening here is these plans are adding superfluous and questionable medical diagnoses to drive up that risk score and get higher payments from Medicare. The theory behind it is if having condition X means you cost an average of 10% more than an average beneficiary, and every person who buys an MA plan from a given company has condition X, they get reimbursed 1.1X the average. They are adding these dubious diagnoses to drive that number ever higher.
What this has led to in practice is MA costs the government much more than standard Medicare. And this is despite the fact that MA plans tailor their offerings to attract a healthier than average population, but then they goose the risk score so the financial effect is the opposite.