Noto didn’t spell out why charging a subscription fee to everyone would conflict with these traits, but here’s what’s implied.
Fewer Tweeters – Twitter breaks news first because anyone that sees or learns something important can instantly share it. Charging would deter or disenfranchise some people from tweeting, so news and commentary would slow because it would have to wait until it was spotted by someone who pays
Thinner Coverage – Twitter has eyes and ears all over the world because it’s free. But even a relatively low price like $5 per month would be prohibitively expensive to some people, especially those in the developing world. Charging would thereby weight Twitter towards the opinions and perspectives of people and places rich enough to pay… More at TC.
I wonder how many accounts on Twitter are not created by real people. Something tells me that the number is very high and Twitter would not want to do anything to reduce perceived subscriber numbers.