There's nothing specific against one-man teams, though it has been consistently said that being a rogue or an outlaw or anything of that nature should be the most undesirable status.
In the case of "we must be a team", there is not necessarily a "must", but in case most of us have forgotten or never bothered to read the BM frontpage...
"BattleMaster is a team-oriented browsergame merging strategy and roleplaying."
Woah, that's even the first line! Later we have...
"Some key points that make BattleMaster different from most other games, both browser-based and not:
* you are a member of a team from the start, and don't have to start out on your own."
Man, the frontpage cites the word "team" twice. That's actually something, considering "low-fantasy", and "middle ages" are mentioned only once, and "nobles" is also mentioned twice.
