You've probably seen a developing trend on the PGA Tour in recent years, and it's meant a whole lot of unmarked golf balls on putting greens. Sometimes you'll see a player hit up on the green and not run up to mark the ball, even if another player in their group still has to play onto the green. It's almost like they're creating a backstop, or some kind of ricochet spot, with their golf ball.
If this bothers you, you have probably also seen #ProtectTheField as a hashtag (or something like it) on Twitter.
Under the Rules of Golf, this behavior could -- and I emphasize could -- be against the rules and subject to disqualification. All of this is covered under Rule 22-1 of the Rules of Golf, and it deals specifically with a ball interfering with or helping another player.
22-1. Ball Assisting Play
Except when a ball is in motion, if a player considers that a ball might assist any other player, he may:
a. Lift the ball if it is his ball; or
b. Have any other ball lifted.
A ball lifted under this Rule must be replaced (see Rule 20-3). The ball must not be cleaned, unless it lies on the putting green (see Rule 21).
In stroke play, a player required to lift his ball may play first rather than lift the ball. In stroke play, if the Committee determines that competitors have agreed not to lift a ball that might assist any competitor, they are disqualified.
Obviously we haven't seen a penalty on the PGA Tour under Rule 22-1 in recent memory, suggesting that the Tour doesn't find these instances of backstops to be blatant or egregious -- no matter how it might appear to a fan watching at home. These guys are extremely good. They don't need the backstop, and, frankly, I'm not sure any of them like each other enough to hand them potential hundreds of thousands of dollars in generosity by leaving their golf balls on putting surfaces to be used in an untoward way.