It would help if you pointed out when these changes happen and who is making them. I very rarely see any admin change the map for any reason, and while plenty of players clamor for Arena I've only witnessed someone actually change it to Arena twice in the last two months.
Was this the specific instance to which you were referring? It took place on 8/19 of this year, and the only reason I have a screenshot of it in the first place is because this particular admin decided to do it twice in a row. After changing the map to arena (no poll) and letting it play out, he changed the map back to arena again the round after that before leaving.
Other things worth noting:
1.) As you can see from the screenshot, at least one person is actively objecting to his decision of changing the map in chat.
2.) At the time, this admin was, if my memory serves me correctly, playing heavy cavalry (Green Corazina Armor, Cataphract Horse, Sugarloaf Helmet etc) and probably benefited considerably in terms of score from the map change.
3.) This particular admin has a penchant for using admin chat for non-admin related business. If you have a reason or opportunity to go back and check the logs relating to this incident, you will likely see at least one or two instances of it in this case alone.
The reason why I am posting this is not because I have any grudge against this particular admin and want to see his powers removed or something dumb like that; on the contrary, I think that having more people available to deal with delayers and imbeciles doing stupid things instead of actually playing the game is a good thing. I merely feel the same way as Leman, in that I would just like to see certain admins limit their actions to their responsibilities and those alone.