If there are clusters and I'm in a bad spot, I'll bust the hell out of them as long as I can leave the cue ball safe. My goal really isn't to make a ball (though I'll gladly keep going if it does), it's to take care of problems and leave it bad so I can run out if/when I get another turn.
I know slop can be frustrating to the uninitiated, but most slop that I do is part of a two way shot. If it goes, it goes. If not I'll have the CB tucked into a nasty location. Slop works best at pocket speed, so it's not the guys bashing the balls at break speed that you have to worry about. It's when the "slopped" ball goes nicely into the pocket at the correct speed that could denote a dangerous opponent.
A 9 ball player (or leagues allowing slop) should learn that it's called "creativity" and not slop. I slop way more in on 9B because it's part of the game. I RARELY slop anything on 8B or 10B. 9B allows creativity and shots with multiple objectives so you gotta learn how to harness that.
If they are "stopping the CB" they are probably playing to give you a bad leave and just pushing balls, trying to see what happens. There are also just hit em hard bangers and I have no idea about that.
I also don't mind playing off difficult shots (where I control the CB) as slop. This keeps an opponent guessing and for certain opponents it can lead them to entirely melt down. There are also shots I like to call "free rolls" where you try to make it, but if it doesn't go, your opponent is hidden. You have a shot at the next highest ball should the "free roll" fall, but if it doesn't, the opponent better be able to kick or jump well.
+ On the two way shots not being slop. That hedging is my favorite part of the game and something I miss with 10 ball.