NXP makes you sign an NDA to use their secure stuff.

The purpose is anti-competitive, preventing NXP's competitors from learning how the devices work. These devices often have advanced hardware and firmware countermeasures.

The secure modules are considered weapons technology if they're allowed to be updated after sale; the company is responsible for tracking each one, they're impossible to ship overseas, etc.

It's not suspicious, it's SOP. Choose between open and secure, or make your own silicon.

Trade secrets are not 'anti-competitive'.

Pretty much all of the providers of secure hardware are like this because they're all reliant on security by obscurity. They rely on keeping secret things like their instruction set, register locations, what countermeasures against intrusion they have, etc in order to make it harder for a hacker to compromise them.

> in order to make it harder for a hacker to compromise them

Keeping implementation details secret DOES make it harder for a hacker to compromise them. When used as a defence on top of a decent security infrastructure. "Security through obscurity" is when a company only uses the secrecy as a defence. This is not true:

> they're all reliant on security by obscurity

They're generally reliant on some secure and proven methods of security, with a layer of design obscurity over the top (and in practice as others have pointed out, they don't keep the design secret for security reasons, they do it for commercial ones).