The big issue is that SolarCity is barely hanging on while Tesla has been on a tear (at least PR wise), so hanging an anchor around its neck seems silly. Everyone was expecting Tesla to need to raise cash to scale model 3 production, so this wouldn't have been news is SolarCity weren't in the picture.
I think the thought process is if they needed cash so badly they shouldn't have just bought a bottomless money pit.
Yes, but SolarCity is losing $300 million per quarter right now. Who do you think will pay that bill?
Tesla will, now that they "purchased" it. In fact, some companies sell themselves for for NEGATIVE dollars (aka, we'll pay you to take this failing business off our hands). Becoming the "owner" of a failing company means that you now have to pay for it.