I would advise that if it's a post-offer day for applicants holding offers, then the applicant really should go. And if the applicant can't they should let the Admissions people know - both central university admissions and the Admissions tutor in the Department. Just a simple email will do.
If the department is going to make admissions decisions based on attendance then it needs to be clear that is what is happening, and it needs to ensure that it has processes in place to deal with the access issues that raises. Are you expecting people to travel the length of the country and be there by 10am? What about students with disabilities? What about students with no money? The main effect of a university in the south of England erecting a "be here for 10am or else" barrier is to make it virtually impossible for northern applicants to attend without buying a night in a hotel and/or a peak ticket: good luck with making your case that isn't an access issue (and before anyone says it, Oxbridge provide accommodation and will fund travel for disadvantaged students, although it isn't well advertised and does act as a deterrent to some applicants.
In practice, if a department invited applicants to an optional applicant visitor day and then made admission decisions, of any sort, on the basis of attendance then it is in breach of my university's admissions code, and I bet everyone else's. There's a whole stack of things we have to do when an interview is determinative which aren't done during AVD one-to-one's (and when we are doing determinative interviews we clearly signpost those and add in the extra formality and record-keeping): slyly using AVD encounters or, worse, attendance/non-attendance would be completely forbidden.
We use AVDs to market - we're hoping people will like us, and the one-to-ones with academics are part of them - and to possibly weed out people who decide for themselves that on reflection they don't like us or our course. The attendees are offer holders who (normally) have more than two offers to choose from, and we run the AVDs in the hope that we will encourage more strong students than might otherwise do so to come to us. That's it. We might piggy-back formal interviews for non-offer-holders onto the same day (for applicants for whom for whatever reason application don't tell us enough) but those students go down a different track and see different people.
If an AVD isn't listed in the course prospectus as a formal part of the application process, and you have reason to believe that your application was disfavoured because of non-attendance, you should complain. Universities should not be erecting "optional, but nudge-nudge we'll use it anyway" barriers.