Interjection: Well, at some point most people, no matter how much they love Torah, have to leave the Beis Midrash to support a family. But that aside, the law isn’t (and never will) force everyone to join the army/service. They’re asking for a certain number. These people who can’t live without a day of learning will still be able to learn anyway.
Once again, if I commit a crime and then run to the Beis Midrash and they come to arrest me, am I being jailed for learning? No, I’m being jailed for the theft/assault/whatever I committed. They might have to drag me out of the Beis Midrash to get me, but I’m not being jailed for learning. Same thing here. People would be jailed for having violated the law (not completing service). Not for learning.