Many of us who enjoy Vanilla would like to have Vanilla servers active and full. By not allowing character copy/move and TBC must be played with a fresh level 58, many good things happen with no 'real' downside at all. First, Vanilla character will remain with all its mounts/achievements, waiting for you whenever you want. Next, TBC started on an equal footing for all -no AH cornering.
TBC can now be enjoyed by all players old and young, new and those who loved it in the past equally.
Achievement/title from Vanilla character could be carried over?
Edit:
If classic servers are about the experience of an expansion long gone, you would start every one at 1 (vanilla) and 58 (TBC).
If classic servers are about progression (so they compete with live), then you would allow for, wait for it, progression.
Lets ask a question: What about players who have NOT played classic Vanilla and would like to play TBC? How would/should they do so? At level 1? Or a template level 58?