The concern is if you own a domain that you don’t own the trademark for.
Twitter owned both the domain and all trademarks so it’s a non issue.
Basically in fairly limited circumstances you can have your domain taken off you if someone else holds a matching trademark. Saying you are selling it can go some way to demonstrating you are not holding it in good faith.
I had registered trademark, and when you register one they ask you what are you doing. I was selling clothes, shoes, fashion stuff. I wanted to get the domain with that name, and it was already taken by someone who sells industrial manufacturing machines. Heavy equipment. They also had registered trademark with the exact same name. The idea is that you as a customer won't be confused who is selling something when you enter a shop and see that name.
I now get what you're saying, but then this can be turned against everyday people. Imagine you have a domain that is your personal blog, vacation photos, stuff like that. No ads, nothing to sell there. Then you receive a message "We from Microsoft/Sony/IBM/Amazon are willing to give you 10Mil for this domain, because we have plans for it, if you won't use it for something bigger ofc" And then show your reply in court saying "He want to sell it". Even if you run a successful business, they can troll you and say "We offer 1bil - you and your family will not have to work for the rest of your life", and then say "His business is fake, he is selling the domain". I don't know if this is possible but if it is, then the whole system is wrong I guess.
Being willing to sell isn’t the only test, otherwise domain squatting would be very difficult. You can generally accept these offers without much risk if done via a lawyer (I’m sure a lawyer isn’t strictly necessary but beyond 5 figures is likely worth it!)