The forum is not actually against alt accounts however the forum frowns at alt accounts. Basically if you disclose your alt accounts then you are actually transparent and not trying to cheat. On the contrary those that don't publicly disclose their alt accounts are actually looking to cheat in signature campaigns and contests.

Basically if you are busy enough managing more than one account would not be easy thats why you see high ranked members with alts disclosed have their alts with low ranks.