If the discount is locked behind vet I think it's fair game. M20 isn't a very meta pick especially in team games, bringing a bonus to not only building it but managing to keep one around might bring it out of the shadows a bit.
This is not a question of fair or not, but more of a mechanic that was strong enough to be consider doctrinal for Ostheer and is vanilla for USF, the only drawback is a cost of 10 more ammo. And I can assume that even if people do not use it ( like the riegel mine while it belongs to one of the most popular commander) it is still god damn powerfull, and especialy in team games where mine are easier to value and protecting flank from Panther/T-34/85 rush is important. And when one of those hit this type of mine (often deep in ennemy territories), it can't drawback and often just die (because flanking isn't something that you support with AT-gun or slow infantry). And putting this mine with M20 is easier than with HT due to its greater view range and speed/manoeuvrability.
Once you understand the power of this mine, you understand the power of the M20. It is not a weaker 222 but a real infantry support vehicule and I'm against anything which could turn it into another brute force LV or cheaper mine.