Why do you get punished for capturing a base with your teamates?

I was the first to get on the capture point, but then my teamates came. After the base was captured i only got 3k sl instead of the full 9k.

You get a reward based on how much of the capture you contributed to. So if several players enter all at the same time it’s equally split (though i think it’s a bit higher rewards than just straight split but i’m uncertain). If a player enters and helps with only the last 10% they only get that small part of the reward.

also, dont leave the cap point before its fully captured. you will still get some reward if you die on the cap while capping it, but if you leave the cap before its fully captured by your team, you wont receive any reward.