> What's the point of submerging in water if we're just going to let it dry? What's happening there?

You really want to avoid sudden temperature shocks that can cause cracking, which is why you first soak the earthenware in water. The water acts like a buffer for the temperature changes as the water in the clay will never exceed boiling point of 100°C. The thin layer of oil on the surface will get hotter, then polymerise and make the pot more hydrophobic, less likely to stain, prevent the unpleasant taste of raw clay, etc.

> When seasoning cast iron,

The metal expands when it's hot. Oil can diffuse into the gaps. When the pan cools the metal will shrink and grab onto the polymerized layer. Overall the heating/cooling is like scrunching up / unfolding paper.

You want the metal pan to be as hot as possible for the maximum amount of expansion, which is going to result in at least smoky oil, if not burnt carbon. Domestic oven / burner can't really get hot enough (it's fine, but people still have problems with the seasoning cracking or failing to adhere), however, a gas BBQ outside is much hotter. You can get a much better seasoning by really getting the pan hot before adding the seasoning oil.

Earthenware has a very different expansion/contraction issue compared to metal. Earthenware is already porous, so lots of little handhold grips for the seasoning to adhere into, no need for expansion/heating. The temperature only needs to be hot enough for the oil to polymerise, but not too hot or too long otherwise the earthenware will dry out and crack.

> Is there any logic behind using olive oil besides "authenticity"?

No logic. Other people use animal fats or other vegetable oils.

You can play around with using a drying oil such as linseed to form a hard, tough polymer film. Or you can use a non-drying oil such as olive and it's just fine.

Extra virgin olive oil isn't the best. It contains lots of non-oil stuff like proteins that make it tasty, but will also prevent it forming a nice tough barrier. You want to use "refined olive oil" or any other refined food oil.

> why the manufacturer would ship this tagine uncured and unseasoned?

Cost and time. It's tradition to season it yourself so you get that local family flavour, so manufacturer will allow for that and save money on an additional step.

Mold and rancid oils. The manufacturer has no idea how long the product will sit unsold on the shelf. Any residual water + oil = microbe food.