Where do you draw the line constituting illegal behavior? There's no way you can do it objectively and only controversy will result.
Example: Tiger Woods hits a bad shot on hole 72 at the US Open but manages to finish with a 1 shot lead afterwards. He lets loose an obscenity under his breath, but there's no mistaking what he said and every camera in the world caught it. A senior rules official who's had enough of Tiger decides to get him disqualified because in his opinion, swearing has no place on the golf course and he doesn't want the kids watching on TV to imitate the behavior.
Example 2: Jim Furyk is in contention on hole 16 at the waste management open, someone yells an insult to his mother during his backswing and he angrily yells for him to be thrown out. A rules official considers any such interaction with a fan to be unsportsmanlike conduct and gives him a DQ, costing him the win.
Example 3: Bubba Watson hits a wild drive and finds his ball against a tree root. He attempts to crush the shot onto the green but nearly breaks his wrist instead and releases the club, sending it flying onto the green. A strict "no throwing clubs" rule has been instituted with zero tolerance because it has no place in golf and Bubba is DQ'ed from the event.
Example 4: Freddie Couples has an opportunity to win a Masters title but hits a hook on the final hole, and drops his club in frustration. An official in the club decides it's such a great story after he went on to win that they declare him the winner and overlook the infraction because he's a class act most of the time. This being the Masters, nothing the USGA, PGA, or R&A can do will make them change.
Example 5: The first woman qualifies to compete in the Masters. She plays terribly despite her incredible length and racks up high scores on the front. Bob Barker begins to heckle her a bit because none of her pets are spayed or neutered, and it ends up getting ugly. Bob really starts asking for it, and she finally loses her cool and pops him in the jaw. An all out brawl ensues, resulting in her disqualification.
Bobby Jones had the attitude that the game should police itself and players build their own legacy. I don't think players benefit from bad behavior like this and I think they suffer from a poor image if they do it often.