It's a fine idea.
Maybe the national venues are selected with that criteria but it's not practical for some clubs that have limited space. Maybe this topic is then more relevant for club HFT shoots where strings do cross but most clubs take their rulings from UKAHFT.
30 targets is a small amount of shots anyway ... so when a shooter gets to shoot one less and gets a zero on a target and doesn't actually get to shoot that target for an innocent mistake I wondered if there was a sensible legitimate reason. It's not going to slow down the shoot because they will be a very rare thing anyway.