I don't know if it's standard procedure, either. I only know that it's morally wrong of your employers, and probably illegal for them to put pressure on you to work in slavery...... And make no mistake, that's exactly what is going on here.
The reason employers get away with treating their employees like dirt, is because there are so many people who take on jobs, and in their desperation to remain in the the manager's good books, (and eventually leave with good refs, even though it's illegal for employers to give you a bad reference) enslave each other by the application of peer pressure..... They'll imply that because they are going beyond the brief of the contract, you are letting your mates down by moaning about it...
Time to find a new job, perhaps?