Well, just as a basic premise, sometimes I get bored and want to do something else. Do I lose the right to change my mind?
What if log in thinking I fancy a bit of poopoo, drop into a crime sweep, then a nonPP mate mesages me and I would rather do stuff with her? Or, more realistically than me having mates who play ED, I drop into a military strike and heyho, there's 8 FDLs - I can't evaluate my situation and make a hasty retreat? How is that in any way a good mechanism?
As for mechanics, how do you envision locking someone in? Can they not low/hi wake? What's to stop them boosting away - will there be an invisible thunderdome sphere they bounce off of? If they log out, can they not then log back in until the timer runs down? What's the penalty for logging out?
Then there's the potential for balance / abuse. What's the maximum ship / instance allowed in ED? If one side ensures they alwats travel with more than 50% of what an instance can manage, what do the other side do?
Here's a silly tactic... your side has a crew of 8 ships that go around attacking military strikes in the system, always together, to ensure all 8 of you instance in the same zone together (let's call you guys Supper-Awesome-PvP-OpenOnly-Gods, or Sappoog for short). An opposing side (let's call them OMG-I'mSoSuperScared-PvE-Gods, or Omgissspeg for short) figures out your tactics, so they come at you with two 800m/s Couriers and 6 SuperFastkill PvE ships. They send their Couriers to find the zone you're in... effectively locking you in there while they do perma-boost laps around the edge of the zone, just wasting your time and avoiding you. The other 6 ships then pair up and demolish three other zones in the system vs NPCs. Congratulations, you've just PvPed your way to a hefty loss, because you're all locked into one zone.
Just to be clear, I have no issue with PvP or being killed in PP conflict, I have never played in any mode other than Open*, and I have openly supported OOPP (for more than simple PvP reasons). I will, however fight my battles on my terms, sometimes I may win the day by destroying my enemy(s), sometimes I may lose by dieing on the field of battle. Sometimes I may win by driving my enemies off, making them flee from my obvious superiority, and that is no less a win in my books. This 'locking in' concept is just completely daft.
*this may change during DW2 if everyone else is in the designated PG.