It's not forbidden. It's just not recommended.
If you want to look at "Tradition," standing was the customary posture for reception of the Eucharist for the first millenium and some.
So, kneeling was an "innovation," and standing is reverting to the ancient practice.
No, it's more serious than that -- kneeling for Communion is ILLICIT, except under extraordinary individual circumstances as identified by the chancery of a diocese. (BTW, they made no exception for Indult Tridentine Masses! They're covered, too!)
The Bishops' Committee declared that kneeling is "not a licit posture for receiving holy Communion in the dioceses of the United States of America unless the bishop of a particular diocese has derogated from this norm in an individual and extraordinary circumstance"
You can say that the Committee doesn't have the proper authority, but the bishops of the USCCB have delegated their authority to it, and until they show enough backbone to wrest that power back, the Committee's ruling will stand.
Not only that, but Cardinal Jorge Medina Estévez of the Vatican's Congregation for Divine Worship agreed in principle with the Committee's ruling that kneeling should be illicit, but thought that a "clarification" should have been added not to deny the Eucharist to Catholics who did illicitly kneel.
And don't think that the modifications are going to stop here. Probably the next thing to happen will be for kneeling during the Eucharistic Prayers to be declared "illicit." The GIRM originally posited that standing should be the norm, but in 1969 and 1995 the US Bishops amended the GIRM so that in the US kneeling during the Eucharisitic Prayer would be the norm. But that debate is far from over.
So talking about kneeling or standing is foolish. It's not the kneeling or the standing that causes us to remember Christ Jesus' sacrifice. We celebrate the communion (eucharist) to remember what Jesus did on our behalf. So talk about this, not about whether one should kneel or stand while remembering the sacrifice that Jesus went through on our behalf. You have just destroyed the very reason for the communion (eucharist), "Do this in remembrance of Me". --Jesus. You can't remember Jesus while arguing about kneeling or standing while receiving the elements of communion (eucharist)