Spectators do not cause server latency increases in any practical manner which could affect gameplay. Correlation is not causation, not to mention there is very little correlation between spectators and latency increases for players that is not purely anecdotal or other void evidence. The internet is a great example of why the general public should never be judge, jury, and executioner. Spectators are a scapegoat, to put it simply.
At the very least allow a few select players to record strategus battles from a spectator view if they apply/register on a thread if you're going to be ignorant and ban all spectators with the excuse of "lag".
If spectators are using player slots, that is another issue and it should be programmed into the strategus servers to create priority for participating players who are not spectators. There is no excuse for this feature not existing other than "chadz is lazy". In the meantime an administrator should be online actively kicking spectators in the event of them taking up too many slots in the server.
The best compromise: Create a thread which states the users who record battles and upload them to youtube with their channel URL, in addition to the users who will stream battles and the URL of their stream. Any other spectators will be kicked.
