Because the system has to reserve a maximum amount on your account on tap-in, and then only confirms payment on tap-out.
That maximum could be a lot of money depending on how far the scheme covers
The entry charge doesn't
need to be more than the highest fare; that's a choice. Returning to the NS scheme, their "boarding fare" is €20 but Maastricht-Groningen (almost as far as you can go) is €30.94 2nd, €52.53 1st. Having a much higher highest fare would be difficult to cope with, so something (what I euphemistically termed "fares reform") might be thought necessary to get that down.
Being able to get on a train having paid for only part of the journey is not unprecedented - there's lots of ways you can do that now. The only new factor I can think of is that, when your "ticket" is checked, your destination is whatever you say it is at the time. But no doubt things like this can be coped with, by suitable system design.
