Why should they do that?
To rebuild trust in the altcoins they have added and to boost the altcoin-economy. If they know that a certain coin is very dubious, they should not add the coin otherwise they are supporting the scam. Yes i know, they want to make quick profits with new pump & dump coins every week, but in the end they are destroying the whole business because nearly no one is trusting them or altcoins anymore. In the long run it is more profitable to convince the average Joe that Bitcoins/Altcoins are a good investment, than to rip off the relatively small Altcoin-Community.
The bussiness is not being destroyed just because you think it is being destroyed. Just accept the game as it is. Most coins are useless anyway and are just p&d. Even bitcoin is not very useful outside the speculation and trading.