Because it prevents lock-in? How's that awful?
Come to think of it, even I am unsure about choosing between free market capitalism and having a bunch of old-age senators writing laws about something that they don't understand (considering that they're clueless about the feasibility of implementing what they legislate at the technical level).
2) It opens the door to litigation. "I told gave <web service> <piece of data> and now I can't get it back easily. Give me money". Don't laugh now, because we all know that laws designed to protect IP tend to be abused (patents, copyright, etc.). I'm sure that Europeans will set the precedent for this one with their tracking opt-out laws.
3) What format should it be delivered in? Can Facebook just give you a 100Mb HTML file with all of the things you've "liked" or does it have to be more accessible? Is a purely machine readable format alright or must it be human readable too?
Of course, it's possible to create a law which gives just the right amount of data freedom to users WITHOUT turning every social website into a horrible Dropbox clone, but there's a very fine line between making legislation toothless and making it pedantic.
If it were implemented as a "convenient" export button, then it would offer a "convenient" way to easily steal all your data. Leave your computer for a second or accidentally forget to log off at the library or apple store? Anyone can come in and download your entire history and pictures and chat logs and who knows what else.
"that's not realistic because it would take too long" or "just ask for password again when you export". OK, but now you've made it even more enticing to steal someone's password since you've created a super easy vector to then gain access to all their info, possibly nicely organized as well. In this sense its kind of nice that its pretty difficult to get old info off of facebook currently.
Secondly, what counts as social network data and what doesn't? Should amazon be forced to provide you with an export of all your purchases so that another web store can offer you as good suggestions and prevent lock-in to amazon? Arguably amazon (or any web store) has already proven to better use this data, and it's as real or more real than facebooks data. How about my Starcraft stats and battle.net chat logs? I know, let's have a long series of litigations to figure this out, in the meanwhile creating a scary and perhaps prohibitively expensive proposition to any new startup that plans to use your prior behavior/data to enhance your future experience.
If you left your computer, then you have bigger issues to worry about (i.e., it being stolen, searching through your emails for CC info, etc). In other words, the malicious use of an export button is no bigger of a problem than it already is to leave your belongings unattended in a public space.
> But now you've made it even more enticing to steal someone's password
It is already enticing to steal someone's password because more often than not people use the same password across services anyway.
I can see where you're coming from on your second point, but I'm of the opinion I should be able to take the data I'm putting into a service (reviews, chats, messages, etc) with me -- it doesn't prevent the service from still monetizing that data (through ads, suggested products, etc), and if anything it continues to give the service an incentive to make their platform better so I would want to stick around with them only because of their superior recommendations and whatnot.