On one level, saying "why has Apple locked down my iPad so I can't run whatever code I want?" is a bit like saying "why has Krups locked down my coffee maker so I can't use it as steam energy source to power my lights?" or "why has Bosch locked down my washing machine so I can't control the RPM of the centrifuge for analysing soil samples?". The iPad is a consumer device sold with the purpose of performing particular functions specified by the manufacturer who will support and guarantee it for that purpose. If you want to hack around with it in order to make it perform some other function, then that is (arguably) your prerogative. But it's not necessarily a failing of the manufacturer not to have facilitated your hacking about.
Now, the situation is admittedly a little more complex in the case of an iPad because, just like your DVD player, graphic calculator, electronic keyboard and various other devices, you might argue that what you have is *technologically* a multi-purpose computer. But that doesn't mean that *conceptually* it is intended to be a multi-purpose computer. The distinction is maybe just a bit more blurred with the iPad than with other devices.
Personally, I don't really see the grand purpose of jailbreaking an iPad. After spending a day battling with my PC over the graphics driver being incompatible with the bluetooth driver or the antivirus not being able to update because of too many flibbles in the patch server or the printer software exiting unexpectedly because I waited the incorrect number of milliseconds before pressing the "Scan" button or whatever other spuriosity one might encounter in the course of an average day's computing, I'm quite happy to sit on the sofa with my iPad at the end of the day and have an hour or two away from that nonsense. That's what it was designed for. I'm quite happy in the knowledge that if I really wanted a tablet for "hacking about" or doing something that the Apple-approved iPad software doesn't do, then I could have bought something else instead.[*]
And I suspect that most iPad users fall into that category.
Now, Apple probably don't care terribly much that jailbreaking exists, provided that-- just like using your washing machine as a centrifuge for your home-grown chemistry lab-- it clearly has the perception of being "a bit of unsupported hackery that the user makes a conscious decision to indulge in". If it became so mainstream that it prevented Apple from selling the iPad fundamentally as a "consumer ecosystem" as intended, then they might care more.
[*] P.S. I should say that I am also a programmer and have a few iOS apps in the App Store. But even as a developper, I don't find the idea of "going through a manufacturer-approved procedure to develop for a particular device" as being terribly terribly shocking-- especially when (unlike, say, console manufacturers) Apple actually make the procedure very accessible to small developers.