I think most people's antagonism towards solo groups is due to two things: principle and altruism.
A solo is called solo - therefore play it alone. That's a fair charge, were it not for PNAPs, should PNAPs not attack together? They certainly organise to defend together. Extend this to a PNAP chain and you have yourselves a group.
A comment was made then about affiliation and responsibility. I completely agree that you choose to play solo because you REJECT responsibility altogether. After playing on a team of 20 with mass inc, wars and politics, it's incredibly attractive to go mercenary and look out for number 1. Yes, it is very selfish. But I hope you can understand why it is appealing to so many who choose that path. So many benefits and so much freedom to do what you want and play how you want wwithout responsibility or banner.