International Institute of Cricket Umpiring & Scoring
Pitch damage

The pitch is the area, 22 yards (20.12m) by 10 feet (3.05m), on which much of the critical action takes place. It would make a mockery of the game if players of either side were allowed to damage that area to suit their own ends. It is, though, quite impossible to stop some damage. Batsmen take guard, play their shots and start to run while on the pitch; after delivering the ball the bowler follows-through on to the pitch; fielders go on to the pitch to field the ball. All these actions are part of the game, you cannot stop them. The Law says that players must not cause avoidable damage to the pitch. Umpires must be clear what is meant by damage and what is meant by avoidable.
The Law does not say that damage has to be immediately visible. If 1000 people walk in line across a mown patch of grass, a path will appear. If one person walks across it there will be little, if any, visible evidence, yet damage must have been done by each person or the 1000 people would not have created the path. The same logic applies to a cricket pitch. Damage does not have to be immediately visible to be real. Mere presence on the pitch is evidence of damage being caused whether wearing flat shoes or shoes with spikes. That damage becomes an offence as soon as it is judged to be avoidable. The striker has no option but to play his shots and start his running while on the pitch, but he must get off it as soon as practicable. There is no excuse for his still being on the pitch when he completes a first run, and no excuse for going back on to it for any subsequent run. The non-striker has no business ever being on the pitch.
The bowler has little alternative but to run on the pitch in his follow-through. Where his feet land, time after time, he will create a mark that can be exploited by bowlers, usually slow spin bowlers, with the necessary level of skill. It would not be in the best interests of the game if an area in the middle of the pitch were allowed to become rough and potentially dangerous. For that reason the bowler in his follow-through must avoid an area defined in the Law and called the protected area. Though extremely important as regards the bowler, the protected area has no other relevance. You will sometimes hear people talk as though batsmen and fielders must also keep off the protected area. In a sense that is true, but only because the protected area is part of the pitch. There is no specific offence related to batsmen and fielders being on the protected area. As with any part of the pitch, unless they need to go on to it to play the game, then they must keep off it. Not only must the bowler avoid completely the protected area, he must get off the pitch as soon as practicable.
Read more about Laws 42.11, 42.12, 42.13 & 42.14 (Fair and unfair play) at the MCC website