I get that it's annoying - but it's a business decision. They're entitled not to honour the guarantee because they can't check the fit or wear in store.
Obviously, they've costed it out & decided that they'll lose more money by replacing shoes than they would by refusing & losing repeat customers.
I'm going to be cynical here, but my PFB had his first few pairs from Clarkes because late XFIL had a thing for wanting to take his dgs for his first shoe fitting, which I was happy to go along with.
They offered to fit 'your other little boy' with 'crawling shoes'. Dd1 was about 14 months, wearing her brother's hand me down clothes & didn't need shoes to crawl in, so I silently rolled my eyes & went elsewhere next time.
All dc managed fine in supermarket shoes from there on in, or by trying on more expensive ones.
Anecdotally most of my friends were the same - they abandoned Clarkes fairly early on because they were over priced & not all that great.
So I suspect Clarkes know perfectly well that they lose customers as parents have more kids & look for cheaper but just as good alternatives.
Their business model is more about new customers than about retaining existing ones, so they aren't that fussed if they lose your future custom.
& their arse is covered in the T&C, so it makes sense to them to risk pissing you off.
I'd also be surprised if they survive this.