You need a quick history lesson, and I don't think it's only you, so I'll go over some of it.
In 1965,
Guy Williams was supposed to be the lead, the hero character, of
Lost in Space. He took the job with the understanding that it would be his show. But before he knew it, a supporting actor leapt to prominence and became the star. Guy Williams suffered the humiliation of changing places with a supporting actor.
The industry noticed. TV's leading men and their agents took a look at this, and in some cases they asked for contract clauses to protect themselves from the same fate.
On
Star Trek, William Shatner was hired to play the lead, a role he rightly expected to keep, but it quickly emerged that Leonard Nimoy was getting a lot of audience response. It looked like the same situation as
Lost in Space was shaping up. Shatner and his agent asked for and got a minimum of
equality with Nimoy's scripted line counts. Not supremacy for Shatner, just nothing less than equality. This not only protected Shatner's hard-earned career, it also had the beneficial effect of protecting
Star Trek from any danger of becoming
The Spock Hour.
TV stardom is a huge achievement for an actor, and it can be lost if it isn't looked after and protected. These are facts of life. Successful people in general want to keep being successful. But Shatner's critics paint a picture in which he alone, in Hollywood, was concerned with his star status (yeah, right). And he alone was the only actor in the industry to wear a hair piece (six of the regulars on TOS wore hair pieces, that we know of). But Shatner, oh, he was the great villain of show business.