Are all merchant terminals PCI compliant?

No. The PCI compliance standard requires, among other things, that merchants and processors encrypt card data and protect databases with firewalls and other anti-intrusion measures. They cannot be modified in the field. The only alternative is replacement.

Hypercom (Optimum T4100) and VeriFone (Omni VeriFone 3200 and 3750) have chip sets that are tamper-proof and resistant to hackers trying to extract out data such as PINs as they flow through the device.
Older terminals, INCLUDING MANY HYPERCOMS AND TRANZ 380’s, do not have such encryption.

The penalty for non-compliance with no data loss is minimal.
The penalty for non-compliance with data breach is $600,000 and up.

So while getting certification has been costly for manufacturers, and thus their customers, the cost for non-compliance is greater. Companies experience 17% customer churn after data breach vs 2-3% average churn.