Many of the employers on the CDS have 2 companies and move staff around to the second company when the 6 month contract is up. Employment law in Spain favours the worker. My hairdresser stupidly gave an indefinate contract to an employee after only a few weeks. She was not there all the time and other employees said the guy had been seen taking coke, not entering some of the transactions etc. She caught him taking drugs and sacked him immediately. He went to employment court and won! Mainly because she had not taken the correct procedure in dismissing him. She had to pay him 6000 euro and he only worked there about 2 months.
I think the short term contract is the reason why customer service is so bad in Spain. People are employed for the high season and they know there is no chance of staying on after the end of September.