Or the inner/outer latch is standard equipment with the casing. Why would the Enterprise crew deliberately remove or disable the inner portion of the latch mechanism, just because the casing was going to be used as a burial coffin?either Spock had an in-built release switch (which would be stupid, since he was dead) which broke the airtight seal,
But why would it have an inner latch in the first place? It's unnecessary, given that only the inanimate equipment and raw materials will be inside the casing.
As for why they would remove the latch mechanism, you answered your own question - namely that it was going to be used as a burial coffin. Burial coffins are usually sealed shut with nails, etc before they are buried underground. In the case of Spock, it was important to protect the crew from the radioactive fallout that his corpse would be emitting. I'm not sure anyone would have turned up for the funeral service if there was a chance they'd get sick with radiation poisoning! With that in mind, It's quite possible that they welded it shut (using a laser torch of phaser), and that the metallic inner is made out of lead.
Regardless of what the movie novelization for TWOK/TSFS suggests, or the confusion of David and Saavik on the Grissom, TWOK movie clearly stresses that the intention was to deliver Spock's body to the Genesis planet - intact presumably.