Why is anyone in a porn movie considered a porn star?

A starring role is the most important or lead role in a dramatic production like a film or play. The simplest definition of movie star is someone who has played a starring role in a movie. This often carries with it a sense of fame that comes with playing a starring role, but is not necessarily the case. The star of a little-known indie film is still a movie star.

If someone plays starring roles only or usually in a particular genre of film, then that person is a star in that genre. For example, Jackie Chan is often called an action star. Similarly, people who star in pornographic films are often called porn stars. There aren't many supporting and extra roles in pornographic films, so most anyone who has performed in a pornographic film could legitimately be called a porn star.

I think you have to have a "starring role" in order that you be called a star. This is as true in pornography as it is in regular movies. The stars of a film are the leading cast members. In pornography there tend to be only leading cast members, if you'll forgive the pun.

Are you sure your claim is true in general (that all porn actors are called porn stars)? Or is it that you only hear of porn "actors" who are "stars" to some extent? At least in the example you gave, the label seems justified: according to Wikipedia's article on the person in question,

In 2010, she was named by Maxim as one of the 12 top female stars in porn.

She also seems to have won a clutch of awards, which presumably not everyone who has been in a porn movie has won.

(And BTW, some less-than-stellar movie actors are occasionally described as "movie stars" too.)

Is this about porn stars in general or Bree Olson?

Bree Olson is a big name in not only in porn but in mainstream media. Just like anyone who graduates with a high school diploma, no matter which high school, is called a high school graduate, one who participates in a porn movie is a porn actor.

Traditionally, "porn star" is used by the media to differentiate between an actor in a film and an actor in a porn film. If a porn movie was considered to be just another genre of film, such as romantic comedy or science fiction, then they would probably be referred to as actors and actresses.

In order to clearly distinguish those who (for whatever reason) the media considers to be undeserving of the lofty title of actor, they use the term "porn star" instead. I would surmise that this was the original reason for the term. At some point the label stuck. Regardless of its origin, I would argue that it is most definitely not an attempt to grant a higher status to actors in porn.