Have they 1) decided that Twitter users are not worth the cost of the ads because they don't consume?
Or are they 2) afraid that the content on the website might become so bad that seeing a brand advertised next to it will make the consumers less prone to buying their product?
Or 3) are they afraid of being subjected to targeted campaigns from activists and politically motivated press because they didn't "cancel" Twitter?
I don't think that the user base has changed so much yet, so 1) doesn't seem plausible; as for 2), I doubt that the content has already become so overwhelmingly bad that ads beside it could cause aversion in consumers (assuming these can't tell the difference between tweets and ads- newspapers advertise anything side to side with news on murder and terrorism, nobody objects). So it seems the problem can only be 3)- fear of being targeted by politically motivated campaigns.