Obviously it's a bad thing if the spawn points aren't in "obvious" locations so the player can try to plan around them, but specifically being unable to block spawn points is not a bad thing.
And it's not like this is the first FE to do this. You could not block spawn points in FE9 and FE10 either.
And how do you know that isn't the case right now? Have you played the game? Is it reasonable to assume that enemies will spawn right in the middle of the map?
Lunatic isn't supposed to be "fair" :P