Eh, it's just how the game works. The goal is to ink turf, and the entire map has turf, including the enemy side. You can't just stick to your side forever, you'll only get 50% of the map at best by doing that, so inevitably you breach the enemy's half of the map. When you do this (or maybe before, if the other team is aggressive), you'll run into enemies eventually, but that's just how the maps are, there's no guaranteed way to avoid enemies. So inevitably you'll end up in a situation where you have to fight to escape, either splatting the enemy, removing them from play briefly and allowing you to continue inking turf, being splatted, in which case the opposite is true, or you'll both splat each other, in which case both of you are briefly unable to aid your respective teams. You do have the option to run away, then come back and ink it up later, but that's not always an option (you'll get cornered eventually, you just have to accept that fact). Still, it's possible to Switzerland a whole match, running away from combat in favor of just inking everything, and still win, if your teammates handle the enemies, but someone has to do that.
Splatoon isn't a game where you don't have to kill to win, it's a game where killing is not the primary goal. Kills still matter, just not as much as doing the actual objective. And you can do both. In fact, in my experience spawncamping people during Splatfest, you have to complete the primary objective of inking turf to get to the spawn point and begin wrecking anyone who tries to escape the spawn point.
At least, that's what I think.
The problem is that some maps, like Arowana Mall, have no alternate routes.
If all the maps were open and had sneak ways, allowing for a multitude of approaches, this wouldnt be much of a problem.
Yeah, that's kind of annoying. Maybe if they used Arowana's Splat Zones map for Turf War, it'd be a bit better. Or maybe I just really enjoy that version of Arowana. i dunno.