Why doesn't it? As long as they get paid, it's basically money in the bank, as long as they serve 4 teams (as they are losing McLaren next year).
More to the point, it leaves RB with no where to go...
Because manufacturers need opponents too. Mercedes needs other teams/engines to beat. And ideally they need teams which can do well but not truly upset their own team. Force India and Williams simply lack the means of competing for the titles against Mercedes. Enstone would be different (if Genii pours in money again) and thus a threat. McLaren was a legacy issue and atm couldn't threaten the top teams.
Thus it would make more sense for Mercedes to sign up with a Sauber or Caterham. They get paid (up front?), can pawn off some technology and are not threatened themselves.
Meanwhile, Lotus is not compatible with Mercedes. Lotus as flimsy, sharp, agile cars. Mercedes are heavy, solid, dependable. They don't share any core values.
Same with Red Bull. RB is all about anti-establishment, rebelling, doing your own thing. Mercedes is establishment, orderly, by the book.