Since you said the game doesnt end till time limit I am assuming you are playing TDM/KC. The problem is these gametypes don't encourage movement, sure KC does moreso than TDM but once a team has a lead little incentive is present to encourage mobility.  This is why I stick with domination, SnD etc.. If the other team wants to camp and sit back they will lose no doubt about it.

It is often annoying seeing the enemy team holding down random corners throughout the match, but you have to understand that in TDM players do not have an incentive to move, especially while in the lead.  Players often will remain defensive once they gain a lead as they control the game.  I have played in local tournies that decided to run TDM, (Dumbest thing ever), and we would win matches after the time limit with scores like 1500-1000.  This is obviously an extreme example as playing for money makes everyone use the most drastic and most likely to achieve a victory playstyle, but online players often do the same thing as they want to win.  Random corner campers in objective can go die though...

TLDR: TDM offers little incentive to move with a lead for those trying to win, gametype it kind of flawed when played in a way such as this.

what if camping is fun to them, what if getting to shoot all the moronic run and gunners that dont pay attention to what they are doing is fun to them? I do a little bit of both but ill tell you what, when you pay for my internet and buy my games you can tell me how to play, till then get over it.