Many players would like to have online games as a fair competition where the most skilled (or most determined) players are most successful. Other players buying their way to success by using money they acquired outside of the game could be seen as detrimental to their game experience.
Many games have an item mall as one or the main source of revenue. Those who don't, likely don't want to rule out the option to start this one day.
An item mall means that the game company itself sells ingame items for real-world money to make the game easier. When other parties do the same, they indirectly compete with the game company for the budget of the players which reduces the revenue of the item mall.
As soon as money is involved, the game becomes a serious business.
One problem is that when people paid good money for an item and then you make a game mechanics change which makes the item useless or removes it from the game, they might try to sue for compensation. IANAL, so I don't know how likely it would be that they are successful in court, but your legal department likely has better things to do than dealing with that.
An even more serious problem is that your game could be used for illegal money laundering. Someone could, for example, buy some items from North-American players, sell them to South-American players, and use their money to finance their drug cartel. This could attract the attention of law enforcement and cause all kinds of disturbances ranging from annoying questions over searches and seizes of your equipment up to prosecution as an accomplice.