Why is 'Salt Asbestos Curtain' the Aperture Science motto?

First off, it's not the company motto, it's an ad campaign.

Aperture Science was a shower curtain company producing deadly shower Curtains before they became better known for developing the portal gun.

Basically, you're reading the phrase the wrong way. It's not "Salt Asbestos Curtain", it's "Salt", "Asbestos", and "Curtains". I'm not sure quite where the Salt comes in.

Its not one of their motto's, it was just a marketing line of the products they were selling in 1940, their motto's are:

• Courage is Not the Absence of Fear
• There's a hole in the sky through which things can fly
• We do what we must because we can
• The best damn applied sciences company on Earth (informal)
• A clear picture of the future
• A clear focus on the future

As listed here.

its the market campaign and salt is there because old aperture was built in and old salt mine and when they went drilling to make the place bigger they had to mine the salt so cave sold it

Salt: Aperture Science is located in a re-purposed Michigan salt mine.
Asbestos: "All these testing spheres are made out of asbestos. Keeps out the rats." - Cave Johnson
Curtains: Aperture Science started out as Aperture Fixtures, a company that made shower curtains for all branches of the military except the navy.