You have to remember that this is a "policy" that is being done in the middle of a draw down of US forces - unlike previous times when they've done the same thing, they are being very explicit about it, and letting the recruiters actually tell recruits this on the road.
They did the same thing in the early/mid nineties - cadets and midshipmen, both academy and ROTC, were graduating after the taxpayer paid for 4 years of college and getting commissioned directly into the ready reserve without ever serving a day of active duty. The only difference between now and then is that they are letting recruiters explicitly tell them it's a possibility right up front.
It's hard to drastically pare down the number of cadets that you are accepting and running through any of the Academies for both structural reasons (it's pretty hard to offer a diverse set of university courses with fewer than about 4,000 students) and political reasons (remember, almost every cadet and/or midshipman is there because a US Congressman or Senator nominated them for that slot).
It's a little easier to pare down the number of ROTC cadets/midshipmen, but even for those, there is a bare minimum that you have to keep pushing through the system if you're going to keep the system viable. And unless you're 100% confident that the US will never again have to call up large numbers of troops, you have to keep the system viable.
Now, what happens when the total number of cadets/midshipmen graduating (remember, they all started 4 years ago), exceeds the number of 2nd Lieutenants/Ensigns the services actually need today?
What happens when the projections show more graduates than you need 4-6 years from now, and you're already running at a minimal viable level?
You tell potential recruits that in some defined circumstances, they will be commissioned into the ready reserve.
And if, in 2021, when they graduate, the military does actually need their service because Trump has got us into a hot war after a pissing match with Putin, guess what?
They'll be commissioned straight into the active office corps along with everyone else.