Co-op interview questions

After passing online assessment I was called for face to face interview in store with store manager. my interview time was at 10 am, and I was there at 9.40 and inform about it. my interview started at 9.50 and finished in just 12 mins. The manger was very impressed with my time and asked me about my experience, As I had long retail working experience, I said to him and he said he got no problem with me. As the position advertised was for 15 hours over 2 days, he asked me can I work overtime like 5-6 days and sometimes nights if needed when he is got hours for store, I replied I can do 3 days but no nights as I got young kids to support my partner. at the end he said he got more interviews to do and I should receive the feedback. in a couple of days I received email that I didn't made to the position this time. I want to tell when they are advertising for 15 hours how can they expect to be fully flexible, it is straight forward abuse of personnel so they can give work whenever they need and don't have to pay holidays if I keep working over my contract. Absolutely disgusting.