Here is the thing, people will have a way to get in those sites, now the problem that you present there, claiming it is a safe haven when the methods of entering there are unclear (at least to me), is that these miners will eventually have to warp back out of there, if these miners have no guardians then all that Corp B needs to do is sit at the stations and shoot on sight! Some gankers do that in Jita in Tornadoes, trying to lock down transport ships swiftly to shoot at before they can dock, so what keeps Corp B from sitting and waiting for Corp A to arrive for them to shoot?
Let us not forget that these High Sec sites are only good for LP, the resources you mine are unusable by players, their sole purpose is to drop them in an NPC hauler and done, a reward of LP. I am fairly certain that Corps that want a proper investment of their time will only run these sites to learn how to run operations while being rewarded for it if these same corps do not bother running L5 missions. And even if they do bother grinding up this resource wars sites, what, they’re getting a skin? A ship? We still have no clue what will be available by mining and fending off some rats in a matter of minutes.
And again, if the sites last only a couple of minutes before blowing up (as said by CCP Affinity they will be “on a very short timer”) I doubt that the site will be there for long to protect them, which does not stop Corp B from the possibility of using Combat Probes and immediately warp there once it disappears, that is, if they ever discover the system they are in and wish to basically camp the system to lock them from ever leaving or something… And I heavily believe that CCP Affinity already stated that there are ways of getting in.
Now here is a question or two, if these sites are locked in number of players, how come people complain even without knowing how many people it needs to be full? Will Corp A really bother in filling up a site to protect their members just to acquire some LP in a matter of Minutes?