Obligatory 30 day Rendering due to Resignation

Hi, more power to you all. You're doing a great job and service to all of us who are unaware of our Legal rights and Obligations.
My concern is regarding the 30 day rendering being required from those who want to resign from the BPO company we're working for. I have once talked to an OIC in NLRC and he told me that it is not lawful to impose or require an employee to render any certain number of days if he/she wants to resign so long as that person does not have documents or anything of sort that needs to be turned-over first otherwise the operation would be affected. He even mentioned "there's no more slavery". Is this truly the case? because I'v known friends who ended up getting terminated for refusing to render 30 days thus incurring a bade record on their employment file. pls help po.  

Hi, in addition, just in case I do get an answer to this post, may I ask to be directed to where I can read that part of the Labor Law like which book volume, chapter and verse, please? I really want to be able to read up on everything that I may find applicable.

Book Six Post Employment: Termination of Employment

ART. 285. Termination by employee. - (a) An employee may terminate without just cause the employee-employer relationship by serving a written notice on the employer at least one (1) month in advance. The employer upon whom no such notice was served may hold the employee liable for damages.

(b) An employee may put an end to the relationship without serving any notice on the employer for any of the following just causes:

1. Serious insult by the employer or his representative on the honor and person of the employee;
2. Inhuman and unbearable treatment accorded the employee by the employer or his representative;

3. Commission of a crime or offense by the employer or his representative against the person of the employee or any of the immediate members of his family; and

4. Other causes analogous to any of the foregoing.