The point is that humans want a meeting's clock time to change when they travel to a new time zone, but not when their local time changes (either due to DST or a legal change).

Unless of course not everybody invited shares the same DST patterns, in which case it has to change for some of the participants (how do you choose which?).

Also, the relationship between the datetime the user chose and the UTC time you store can be retroactively changed by legislation.

Timezones change.