Is it plagiarism if I don't follow these guidelines?   What are the penalties for plagiarism?   What if the plagiarism was not intentional?

Plagiarism misrepresents the words or thoughts of others as being one's own.  So, yes, it is plagiarism if one doesn't follow the described conventions and guidelines.  Such misrepresentation is wrong, because in doing so one takes credit for another person's accomplishment.

Plagiarism in college courses does carry penalties. These range from failure for the assignment to failure for the course. Severe or repeated offenses can result in dismissal from the University.

It is the writer's responsibility to make sure that another's contributions are not represented as being one's own, and that proper credit is given.  This means that it is the writer's responsibility to make certain that the conventions are followed correctly and completely.  The person who fails to do so is not taking proper precautions against misrepresentation.  Such misrepresentation is culpable, and carries the same range of penalties as intentional plagiarism.