That's a good question.
Whether boosting is bad is a matter of opinion. It's mostly an argument of morals and whether one person feels pressured into doing it in order to not feel unfairly disadvantaged. Everyone who does do it, feels remorse for their illogical stats; so even they obviously question the validity of doing it.
But why is it bad? Well, that has a technical answer. The answer is that EA's system can't accomodate it. Performance-wise, it is terrible. It makes each server used for only one solo person. When boosting is happening, you can tell. Every other community server has terrible lag spikes, high ping, and are hard for people to find and join.
So, if people are going to do it, it's only fair that all servers get full XP to hopefully drive that traffic to that more legitimate server, that more people can experience at the same time, that EA supposedly encourages.