informal vs formal verb usage

In lesson 1.2 why is the informal form (te llamas) used to ask someone you don't know what their name is?…why not the formal (se llama) since you don't know them?

Formal definitely can be used, but these are people of the same age being introduced by a mutual friend, so informal would be OK. 

There are not many absolute rules about when to use formal vs informal. In fact, it varies by country how much formal vs informal is used.

I live in Southern California, and tú seems to be used in advertising to Spanish speakers. One service station in Bakersfield advertises “energía para ti, gasolina para tu auto”. That sounds disrespectful to me, but in Ecuador I saw government signs aloso used the tú forms. Can't recall the exact wording, but on the highway there was one about driving safely because people were waiting for you, and the “you” was tú not usted.

Went through the Houston airport today and all signs are bilingual. All Spanish was in the informal.

It wasn't the advertisement, it was the directions to travelers. “Terminal A is this way” type signs.

The train sign showed me something I never thought of before.

In English:

Train for Terminal A & B.

In Spanish:

Tren para terminales A y B.

It makes total sense not to substitute a one letter word for a symbol, but it struck me as odd the first second (since I was so used to seeing the ampersand).