If you actually have the system in place to get the data from the card swiper into the appropriate fields in the AuthNet system to fire off a transaction request, I can't see any problems.
If it is a card-present transaction, then that means the customer is right there anyway.. so there's not much difference between that and the customer going to your website to make the purchase.
As far as the 'legality' of it, that's above my pay grade, so we'll have to wait and see what the responses are to that.
Have you considered using the Terminal for manual transactions for this? I understand that using that wouldn't exactly do everything that your own web site might be doing (setting up accounts, logins, etc), but if you're just trying to get paid - without any back-end things happening that your site would normally do during a web-present transaction, terminal might be the way to go.