They are not "street dates" though, they are the Target planogram reset dates.
The Target corporate offices send out planograms to the stores with a suggested reset date (in the latest case for the SW aisle it's 11/19). That way, the majority of the Target stores will switch over to the new aisle set-ups at approximately the same time and 90% of the Targets will look the same when customers shop there.
To ensure that there is product to go on that set of shelf arrangements, Target will often hold back new items so that they have them for the reset date (and don't have the blank spots on the shelves).
Target has always done this, the planogram re-set dates are just waaaaay more visible when it comes to Star Wars toys. There is a bunch of stuff in the backroom (like the Tin Sets and Mace Windu Battlepack) that Target is holding simply for the reset date. It allows them to "refresh" the toy aisle so that once a month (or so) there is new product out there.
It happens all the time with other toy lines too, but they are just not as visible as Star Wars. For example, last month, my Target had the Mattel JLU 6-pack (with exclusive Doomsday) in the backroom for weeks, but they did not stock them until they set up the new DC Comics endcap planogram in mid-October. If you watch the toy section as a whole, you'll notice that every month there is some kind of reset somewhere on the shelves.
I think the Target planogram dates are just way more visible these days because way more people know and use the DPCI numbers to try to get stuff from the backroom. Without having those DPCI numbers, you would never know about a "planogram date" - you'd just walk into the store and find new stuff one day.
I think the dates are more visible too because many frustrated store managers come up with the "street date" story as an excuse not to go get stuff. Despite what those managers say, there is no Target corporate policy against getting something for a customer when asked (if fact, to the contrary they are supposed to do everrything possible to satify a customer with a question).