It's artificial scarcity, which to me is immoral, but it is what it is.
In your example, you have the choice not to buy the license. In the case of Protonmail, they already have your emails, and are already serving them to you, so holding your correspondence hostage if you want to export it all in one go is a bit of a dick move.
that's obviously a strawman. What's being critized is the heroin-dealer model of doing business, i.e. "the first dose is free" but you'll be locked in, which seems increasingly popular among a lot of services that try to compete with the big players.
no because the comparison is in the mechanism of luring in customers, not the substance or the price. We can go with any commodity you feel comfortable with if that improves the analogy for you
If it's true they require you pay to export emails it sounds like borderline extortion.