Not sure about the legality of it TBH, but where the beer is served via a long spout that is partly submerged by the pour, it is conceivable that a previous user may have transferred 'something' via saliva into the glass, which then transfers to the spout and lurks before being washed off by the next pour. We don't sell real any ale,* most often associated with that type of pump and use short nozzles that don't come into contact with the glass contents for our fizzy beers and ciders. Default is a fresh glass every time but no argument if you want to keep hold of your first one, as there is a negligible chance of contamination where there is no immersion.
* I wish we did, but our punters don't want enough of it to warrant stocking it, we were chucking out half kegs every week. 'Spoons across the road that we can't and won't try to compete with on price.
Edit to add: I was pretty sure it isn't a specific requirement in law to use a fresh glass each time, and it seems that's the case. That said if anyone falls ill and can prove the cause was the sort of contamination I've described, you could be in bother, thus the safe practice is to insist on a fresh glass every time for the serve. Nothing stops the punter from keeping their glass and decanting.