I can see how this is an issue for Anglicans–at least for Anglo-Catholics. Also for Lutherans.
But other Protestants do not claim a kind of “validity” for their Eucharists which Catholics are bound to deny. Those who hold to a purely memorial view certainly claim no more for their Eucharist than Catholics grant. This is probably also true for those who hold to some kind of “spiritual presence” view (moderate to low-church Anglicans and high-church Reformed and Methodists), though it’s perhaps a bit more dubious.
So what exactly would Catholics be professing that isn’t true from a Catholic point of view?
I don’t want to push this too far. I understand the reasons for the canonical prohibition–obviously even a purely symbolic Eucharist still purports to be a celebration of the Lord’s Supper, and thus a version of the same thing of which Catholics believe the Mass to be the true version. And obviously in a sense Protestants are professing in receiving Communion not only that Christ is symbolically/spiritually present (which Catholics can also say is true in Protestant Eucharists) but that this what Christ meant when He said “this is my Body” (which Catholics obviously deny). That’s reading a bit more into it than most Protestants do, but hey–reading more into things is kind of a Catholic distinctive!
Edwin