You missed something; the only part of ACID its relaxing is isolation: http://en.wikipedia.org/wiki/ACID#Isolation

"This property of ACID is often partly relaxed due to the huge speed decrease this type of concurrency management entails."

Indeed.

If you have the viewpoint that DB validation is for catching programming errors, not invalid data (i.e. you validate everything before it hits DB anyway), this relaxation can be reasoned about client-side anyway.