Even if you have a CS in place, why on earth would you want to have to train your employees, to field calls and situations from an entirely different service?
That doesn't make any sense at all.
Sony doesn't "need" to share anything. They have already allowed other companies to provide services on SEN without requiring access to PSplus.
The reason MS is ok with taking this on is because MS normally provides this service as well. Their resources means that EA is probably paying them or giving them a cut to use their distributed services and that allows EA to pass the ToS along. With SEN, EA would have to maintain their own servers and services and wouldn't be able to pass the responsibility along.
Sony hires out their CS, the actual increase to costs via contracts wouldn't be that much, and they have no reason to care about training their staff. MS also hires out their CS, so the actual cost to this when you already have CS in place is, I would wager, quite small in comparison to potential profits. The difference is if you are willing to share space, which could potentially eat your own market share by allowing another player to come into what is currently a walled garden.
As for the bolded, this is an assumption, speaking for MS as the "reason" they are doing something other then profit motive is a pointless venture. I think it's as simple as MS is willing to share game service market share, sony not so much.