The maximum party is indeed 5. However, what these premades do is they queue up all at once in parties of 5 and land in the same queue area and mostly, with coordination appear in the same desired battlegrounds.
This is not against the game rules as long as they don’t use soft or hardware assistance like previous existing addons, illegal .exes etc of some kind.
Yes it can be considered unfair. If you have suggestions you should submit it through the ingame suggestion panel too!