Then why have a max TT value?
I'm fine with being allowed to sink whatever TT you like into your gear. But no reason to assign a max at all in that system.
No skin off my snout, right? It's going to be harder to sell if you don't grind it down a bit but repairable items are TT+ not % for MV. Armor is different though, they would have to change the way that works to allow open TT.
On this aside the item has a minimum TT value and it will be somewhere between that and full TT. You can guarantee full TT by adding various residues to the craft but the system knows the actual return on the click and won't announce a global for the value of the residue included in the craft.
This is the only reason that most of the crafted (L) items you see for sale have full TT. Nobody would typically add residue while crafting a repairable item because you would lose the MU on the residue - instead you just craft those and repair them after (if you want to) for no markup.