So your argument is really that DRM is ok as long as the user knows about it in advance?
For the same reason Siri needs a connection to work: there's a lot of heavy processing going on, which your client device just couldn't handle even if it wanted to, that is being offloaded to a cluster somewhere instead. In Siri's case, it's linguistic analysis; in SimCity's case, it's inter-city traffic-flow, economy, and other expansive high-granularity world-simulation tasks.
...or, at least, that's what EA's engineering claimed at some point. To still be charitable to them, but to temper this with pragmatism a bit, I don't think there's really that much difficult processing going on; I imagine it was more that high-end gaming PCs could run everything client-side, but the low-to-medium-end PC market--from which Maxis pulls the lion's share of its sales volume--couldn't.
Shoving the processing server-side means everyone can get the effect of throwing a 3GHz Xeon at their world-simulation, and the total amount of processing required can be decreased dramatically by using player's real cities to neighbor other real cities for inter-city world-sim calculations, instead of having to simultaneously synthesize the behavior of 8+ AI cities to interact with each real one.
To put it another way, I imagine it would be very hard to market a game where the core feature ("new, more granular, more realistic simulation!") degrades into non-existence on the average PC, thus rendering an upgrade from the previous version moot.
Games are not generally expected to require a network connection. If I take my laptop to the coffee shop without wireless I can still play Portal and Anno 2070.
Not so with the new SimCity.
Not to mention, there is an expectation that if you buy a game (especially on physical media) that you will be able to play it so long as your hardware supports it. I can still play Quake 3 for example.
With SimCity, if they ever shutter the servers or sunset the Online Service your game becomes nothing more than a pretty coaster.
Unless you bought Portal on Steam and didn't sign in when you had an internet connection.
The DRM strategy in Sim City is not the same as Steam, and cannot really be compared. EA are forcing unnecessary always-online features into what most people expect to be a single player game, preventing it from being played offline at all, in a thinly veiled attempt to prevent piracy and resale. That would be bad enough, but what makes it utterly unacceptable is that they haven't provided the infrastructure to support it in the short term, and clearly don't have the business model required to support it beyond the first few years.
Why anyone would knowingly support this business practice at all is beyond me, let alone for a game which has had such dubious pre-release reviews and is now essentially unplayable.
I simply said comparing this to a web app is not apples-to-apples.