5 minute siege mode is carefully balanced on an assumption that both teams are trying to capture the flag in the most efficient manner possible. The whole point of attackers busting down doors and gates is to decrease spawn to flag time from what is usually an unwinnable amount to something very winnable. Defenders' ability to efficiently hold up attackers at critical chokes determines their chance of victory. Ladders unfortunately had to be removed because on most maps they allowed attackers to instantly bypass chokes, and setup efficient flag routes right at the start instead of having to fight for them as intended.
Anyone who's out there on the map doing something other than doing their utmost to efficiently fight for chokepoints is basically sabotaging their team because they take a roster spot but aren't participating in anything close to an efficient manner towards team victory, so you are giving the other team a numerical advantage. This is really exacerbated when there are multiple people doing this, being banner balanced together. This is why people dislike you and you get banned, because you selfishly play the game, in a team game you engage in unproductive activities for your own personal satisfaction, and sabotage the chances of victory for your teammates by giving them a huge handicap. You are basically doing the effective equivilant of taking some people off to a corner and holding a longjump competition.
You have a case that siege equipment should be in use in game modes besides strategus battles, anyone who's a fan of siege would love to see it, but the current siege mode is not balance for it at all. The devs have tried to make a new sieeg mode that has engineering (stronghold) and apparently ran into some roadblocks. Maybe the Warband engine patch coming with Napoleon DLC and including upgrades for object destruction will help CRPG devs implement catapults, or maybe some kind of public version of strat battles can be implemented as others have suggested (500 tickets, 15 minutes). But right now you guys can't just go into normal siege and fuck it up for the other 75 players, it's unbalancing and it's selfish, you deserve any bans you get.