Because they make a profit on Pellegrini, whose book value was probably close to 0. The money spent on Spinazzola is a capital expense and will not affect this years EBITDA. This is one of the many reasons why FFP is retarded and can be easily manipulated.
Hard to say if we robbed ourselves here. If he's a solid defender that won't make costly errors and if he can make an impact going forward then you'll take him with 10M. I'm thinking of Lichsteiner who wasn't remarkable in any way (like Spina is at dribbling for instance) but always managed to do what was needed whilst knowing his limitations. If he can be that kind of player for us, then that's a fine a backup.
I don't think I've ever watched him play. Definitely haven't noticed him. He's only 20 so if his highlight reel isn't that impressive you'd forgive him. The main thing for a full-back at his age is to have solid fundamentals.