As to why exchanges keep shitcoins listed, it's mainly because they do still get traded--in fact, that's the only purpose they've ever served. None of them get used as currency, and my guess is that most people don't hold them for the long term. And take a look at Yobit and all the shitcoins they have listed. Some of them seem to be exclusive to Yobit and I'm not even sure they exist outside of that exchange. And Yobit is probably the worst in terms of having coins listed that have no volume and which can't be withdrawn because their wallets are always in maintenance mode.
Also, the definition of a shitcoin is somewhat subjective. Some folks think doge is a shitcoin, whereas others think it's one of the best, and most exchanges have it listed. But when you're talking about a coin like TSL (whatever it is), there would probably be unanimous agreement that it qualifies as one. I've no idea why that BitMart exchange has it listed aside from my guess that there are probably some people who still trade it.