A creature's standard movement speed is its movement on land (the default). The Swim speed is specifically for underwater (movement rules and pg 8 of the Monster Manual).
If both are specified then the creature is able to move on land but is also at home in the water. In this case I would expect a listed Swim speed to be greater than half its normal movement, otherwise there wouldn't be much point.
If a creature has no movement listed then its default movement (on land) would be 0, though a DM may determine it can flop about a bit.
You are right that there isn't a rule the other way around (stating that creatures with only a Swim speed can move at half-movement on land). But this a realistic "disconnect" and pretty much as per real life. I have a walking speed and I can also swim (badly) as many land creatures can. My goldfish has a swim speed but would certainly not be able to move on land!
An entirely aquatic creature would also not be able to breathe on land and would suffocate as per the suffocation rules (which do apply either way around). Some creatures can breathe both water and air, and these are pointed out in the Monster descriptions.