re: I thought it was legal now
It is in the United States – and what you’re referring to is a change to the Digital Millennium Copyright Act. Every three years, the US Copyright Office looks at the Act to consider changes.
There were six main changes that legalised activities that you could be prosecuted for, two of which were:
“ Computer programs that enable wireless telephone handsets to execute software applications, where circumvention is accomplished for the sole purpose of enabling interoperability of such applications, when they have been lawfully obtained, with computer programs on the telephone handset.”
“Computer programs, in the form of firmware or software, that enable used wireless telephone handsets to connect to a wireless telecommunications network, when circumvention is initiated by the owner of the copy of the computer program solely in order to connect to a wireless telecommunications network and access to the network is authorized by the operator of the network.”
So this means it’s legal to jailbreak any phone to use (legally obtained) apps that you wouldn’t be able to normally access (e.g. non-App Store ones for the iPhone) or in order to use a different phone network.
I would also say there were some rather major changes to the DMCA, such as it’s now legal to break DVD encryption in order to obtain short clips for the purposes of education or criticism.
Going back to jailbreaking, I don’t think there’s any hard evidence that Apple is actively trying to stop this. It would be a waste of time as you say, but Apple has never taken action against the Hackintosh community or those that build these machines for personal use. The only people they have pursued for this type of thing in recent years were Psystar, a company that indulged in dodgy practices, including charging for open-source software produced by the Hackintosh community after they slapped on a GUI (which looked suspiciously like another freely available GUI). My own feelings is that Apple takes a similar line with jailbreaking.
One thing I think worth remembering that although in the States, it’s legal to jailbreak this doesn’t mean that there are no bad consequences – although this hasn’t been challenged legally, jailbreaking will invalidate your iPhone warranty. However, it’s very difficult to brick your iPhone by jailbreaking and even if you do, you can restore it to it’s pre-jailbroken state by using… iTunes. I'm pretty sure (but happy to be corrected if wrong) that this didn't used to be the case.