> In practice that means that when you enter US you cannot be planning to apply for immigrant visa.

You are correct about this.

> H1B is also non-immigrant visa, but it is dual intent visa meaning it doesn't have that requirement

You're incorrect about this. The concept of "dual intent" doesn't exist in the Immigration and Naturalization Act. It was created by executive fiat. H1Bs, like other non-immigrant visas, still requires non-immigrant intent. It's different only that it has two carve-outs:

Subsection (b) excludes H1Bs from the "presumption" of immigrant intent that applies to other categories of aliens. Subsection (h) provides that applying for permanent residency "shall not constitute evidence of an intention to abandon a foreign residence" for H1Bs.

So H1Bs must still have non-immigrant intent. It's just that they are carved out of certain presumptions that would automatically establish immigrant intent, which would lead to denial of their visa. It gives the executive flexibility to essentially look the other way when an H1B applies for a green card. But it doesn't confer any legal rights* onto the H1B. The administration can at any time decide that you actually have immigrant intent and yank your visa.