Lets maybe examine how Siri works in the first place shall we? The voice content is distributed to servers and processing center's which in turn process information based upon a very unique identifier.
Scenario 1: While creating the ipad 3, the unique device identifiers were created in such a way that siri processing would be possible or possible in the future. But lets be honest here, the actual system requirements to run Siri are not demanding, a number of online blogs and tech organizations have mimicked Siri functionality on iphone 3s by spoofing unique ID's. Also, it probably wouldnt be difficult to revamp whatever Siri infrastructure is in place to accept ipad 2 etc.
Scenario 2: There have been 85 million ipad2's sold. If the infrastructure is not in place to proces all of the voice data, apple's servers/centers would be bogged down on a constant basis. What would they do given this scenario? Pour millions into developing a larger infrastructure? Or, maybe, they could simply not care. By not caring, they wouldn't have to spend money. In fact, they would be able to make a lot of money. People who wanted that service would be able to upgrade their devices and pass on their old ones or sell them.
Why should a business not be interested in making money? Because they are nice and friendly and they love their customers and want their trust? No. They are great at what they do. The reason they continue to pump out amazing product time and time again is because they play their cards right. And while yes...they don't neccesarily pump a franction of what they could into R&D they still never cease to amaze us. Carry on Apple.