1. Practical. Porn is banned because Stripe doesn't allow payment for porn. In turn Stripe doesn't allow that because the chargeback rate on such payments is very high, supposedly due to the number of people who buy porn because they want it and then get caught by their spouses and claim their card must have been stolen. At any rate, a blogging/newsletter platform is not exactly optimized for porn anyway. The people who want to buy and sell that are better served by other platforms.

2. Theoretical. Porn is not "speech" because speech encodes ideas or viewpoints. The reason freedom of speech is important is because ideas and viewpoints are important, as it's only through the competition of ideas that progress occurs. Defending free speech takes a lot of work, but can be justified by the benefits. Defending porn would take even more work, but wouldn't yield the same social benefits.