Is it ok for recruiters to call an applicant’s former employers for references?

Yes, It is ok but should tell the candidate before doing that - and not all employers says the truth 

That's what the references are there for. To ask about the candidate

Definately YES because former employer knows his or her knowledge, skills, attitute, integrity and accountability and how he or she perofrms better in job. In my organization,02 former and HR referees mendatory before offering employment.

yes, but he should be skillful enough to know what he will hear is the truth or not. It is better to have structured questions

yes

Some former employers do not provide accurate references on former employees, especially if the former employer wanted to keep his former employee, but he was not able to offer him some better working conditions.

The employee need to provide references as per request, if there are no references then the recruiter can make reference check by phone or email to HR dep. 

It is OK but should not be a protocol for that not all employers are unbiased and objective in judegement, getting feedback from former employers is a useful tool only if : employee is placed in a position fitting well to his skills, working conditions are good, management is objective. Otherwise, the feedback may do more hram than good to the candidate.

NOT before the joining, do it after the joining and do inform the employee too.