Jobs lost jobs interviews applying for jobs?

I applied for a job and the end of January end of febuary, I had an interview and it went very well, so I gave my reference numbers and waited to here from them, I waited about 2/3weeks and rung them on the Thursday leaving a message asking to hear weather I was successful or not, I've not had a job a while and was putting so much into getting this job, I got a phone on the Monday and I got the job, the man told me to start on the Saturday and he was looking forward to working with me, i was extremly excited i told everyone, I was traveling home tuesday evening and got a phone call to say that he couldn't give me the job because one if he's previous staff couldn't do there weekday shift could only do the Saturday witch was my shift, so he couldn't give me the job,, I've been devastated every since and I've been thinking, is people aloud to do that? Are they aloud to give you a job and not give you a chance?