Its also possible that Fredo allowed the assassins on the grounds. If they arrived w/ Johnny they would have been recognized after they were dead. Fredo did not slice the assassins throats, a cpl of Ola's men did that, and these men were shielded by Fredo and later allowed to escape.

Perhaps Freddie was told by Roth 'Open the drapes so we can get a good look at what we are shooting, they are just gonna scare your brother a bit, fire a few shots into his room, get him to thinking.' Of course I think too its also possible that they were indeed just trying to scare Mike, and thats the only reason he lived.