If either parenthetical citation or works cited are missing, how is an assignment plagiarized?

If you copied from someone else's work and don't put the work's cited then you are stealing their work, because you didn't really do anything but look up what someone else said/put.

Since informal writing is the act of acting unproperly, or as you do with friends or in text - stating opinions may be quite common. 

It doesn't make sense to use complex sentences, scholarly language, or lengthy descriptions informally. That's meant for more formal occasions, such as teaching a particular subject or discussing with people of high intellectual knowledge in a formal event.