I would suspect that the cost to the business of training zero or low hour temps is quite low and gives them great flexibility. Especially when compared to the higher wages and overtime multipliers of existing staff. I would also think that companies that have a lot of staff that do overtime are getting concerned re the recent legal case on over times impact on annual leave (whilst this will certainly go to appeal).
I was always told to never rely on overtime to maintain a lifestyle, always live within your basic. I know that sounds patronising to those who struggle to do so, but I have always tried to live that way, not that I get paid overtime.
As stated it is a business decision, it is not a right. The statement that interests me was "promised".
Edited to correct speeeeling