Most players consider this an act of theft and rudeness since the original pirate does not get all of the Reputation award for defeating the enemy alone, but rather must share it with the outside pirate who intervened. This theft of Reputation is where the term Rep Stealing comes from.
When on the other hand, some players consider it helping. Say a pirate is almost knocked out and needs some help defeating an enemy, he/she will appreciate the help.
Often, an inexperienced pirate may be unaware that they are doing so and are simply trying to help. However, experienced players take issue with another pirate consistently taking part of their rewards.
One way to prevent this or from being rude and stealing rep from other pirates is to get into a crew together.
Joining or creating a Crew effectively reverses this situation. By being in a crew, ALL members are rewarded and get a reward bonus, depending on the size of the crew (from 20% up to 50%).
Another way to prevent (or cause) rep stealing is by using a powerful gun, such as a blunderbuss, defeating the enemy with one shot, ensuring full ownership of the loot.