It seems to be a trend within the automotive sector and the justification, by some auto makers, is safety and security. Nevertheless, it is interesting to read that other companies are happy to use the increased 'security' to channel customers down a manufacturer approved upgrade path:

Quote
"We don't want to lock the cars and say you can't modify them," Dodge CEO Kuniskis told Carscoops. "We just want to lock them and say modify them through us so that we know it's done right."
- source

The legality of it is an interesting point and I wonder if this is addressed in the sales contract. Pondering as I type, perhaps a solution would be to have the ECU locked from new until the end of the warranty period. The systems are intelligent enough that a program could be turned on during PDI and set to the delivery date, which would then lock the ECU until the warranty period expires. My assumption would be that this would protect the manufacturer but allow the consumer freedom once they are no longer legally covered by the warranty. Should the customer wish to have the ECU unlocked prior to the warranty expiring, they could be asked to sign a document waiving their warranty and the security can be deactivated as a result.