Not underhand at all. And nothing new. When I was nearing leaving school back in the day there were millions of people unemployed and the prospects for young people were far, far worse and more bleak than today’s youth. I did loads of free work just to get experience and break through the ‘cant get a job because you’ve no experience, can’t get any experience because you can’t get a job’ paradox. So I signed up for free work all over the place in many different areas of work. And it all paid off because I applied for 10 apprenticeships and got 10 offers, so sometimes you’ve got to play the long game.
Similarly on zero hours contracts, I effectively worked zero hours contracts, just by a different name, when at uni during holiday’s, so nothing new there. Was a great arrangement, no commitment on either side so if I didn’t want to work one day I didn’t have to. Was perfect for the situation I was in in life at that time. Not perfect for everyone, but good for some.
I’d advise to do the shift, give it your best, make a good impression and then earn the job at the end of it. It’s a competition at the end of the day, not an entitlement.