1. It has nothing to do with "space". I assure you that the Storm2 is capable of running anything the Tour can (at least from a computing perspective -- I'm not sure if there are hardware requirements that the Tour has and the Storm doesn't)
2. You can't really compare OS versions across different devices based solely on the version number, since OS's are built specifically for individual hardware. Is there somewhat of a correlation? Yes...but it's kind of a Nave viewpoint to state that because the Storm is on version .607 and the Tour is version .591 that the Storm's OS is "16 better". The bottom line is that the Storm's OS may have never have been built around the capability to use PTT as a feature...so it hardly matters what version # they're on if they've never intended for it to be a feature in the first place.*
*I'm not stating the Storm's ability to use the PTT feature is or isn't OS dependent, I have no idea...I'm just saying it doesn't necessarily matter the OS version number vs. the Tour's when it comes that feature being available.
3. I actually have found it odd that Verizon would roll the PTT feature out just for the Tour and now Bold 9650, without so much of a mention of the Storm. Seems like there must be something preventing it from happening though, otherwise why not add the feature to all devices that could support it?