This is the slippery slope argument, but if this is done for security reasons, once the infrastructure is in place for immigrants, I don't know why it should not be applied to visitors.
No argument, just a question. The answer will affect my decision whenever I'm asked to travel to the US next and I'm sure I'm not the only one.
To play devils advocate, it's highly likely multiple countries are already looking at this information, this is just a formal way of asking for it to add non-compliance to justifications. It's almost akin to the US state of Tennessee having a marijuana tax despite not allowing legal sale of the substance. It becomes another thing to use on charges / justifications.
and what is prevent locking down accounts to private (the way you should be doing anyways) ?
Many social media sites are headquartered in the US, so private accounts there are completely open to an NSL (or the DHS' equivalent). Information-gathering doesn't have to comprehensively cover every site to yield results.
Anyone non-native born and anyone (even native born US citizen) that has communicated with such.