Never said that. Also Net Neutrality regulation isn't the only thing preventing ISPs from blocking all the websites they want willy-nilly.
What ISPs want is ability to charge more for priority services (which includes but not limited to 'express lanes').
What NN supporters think this would mean is that ISPs will block websites.
Anyone has a right to block any content on their private property. Newspapers can censor articles. TV can only air what they wanna air. You can chose to listen to whichever radio station you wanna listen to. Until the ISPs promise you to not block any website, they have a right to block any website or deprioritize it for that matter.
When ISPs sell you an internet package, a 'feature' in non-NN world they would wanna provide you is: 'No blocked sites' in addition to Fiber, fast speeds, low prices, etc.
Any internet package which has blocked content on it, and blocking wouldn't be done proactively or blacklist-based, but rather like Free Basics was. Only a certain whitelisted sites were allowed. Any such package would be priced lower than a no-blocked sites package. Why? Because fewer people would want it. Discrimination of data also allows investment into the internet infrastructure. It's like Facebook ads are paying for the internet for the poor people.
Imagine this, you find out that people in New York love Mangoes, but there is nobody supplying them with Mangoes(and only super rich people are able to afford Mangoes). You find out that the city of Chicago has a LOT of Mangoes and available for very cheap. So you take your savings, buy a truck, drive to Chicago and bring a truck full of Mangoes to NYC. The question arises, since everybody from rich to the poor want Mangoes, at what price do you sell them?
If you auction the Mangoes to the highest bidder, then only the super rich will be able to afford it, if you sell it at a minimal profit(since lets say you're not in it for making profit) then while everybody will be able to afford it, only a few people will be able to acquire them until you bring back the next batch.
If you want to be able to provide Mangoes to maximum number of people, the smartest strategy is to sell them to the highest bidder, and then use the extra money you're getting to buy more trucks and make frequent trips.
This is exactly what Net Neutrality prevents from happening. When discriminatory services are provided, it allows for more services to everyone. You think that Microsoft will be able to pay to block GMail, where as what will happen will be more like UPS's regular mail vs overnight delivery. Amazon will not pay UPS to ship things through regular mail if UPS started accepting money from Walmart to slow down or block Amazon's packages.