I don't think they don't understand, but rather they don't care. Obama is at least a decently smart guy. After all these repeated infringements by the government, there's not really a chance that he's somehow unaware.
See how the court chooses their words to paint a simple technical action such as changing a user agent to be this inherently malicious thing that only an expert hacker would ever do?
I believe that is shared by everyone in government. They just do not know what's going on, and they fear it.
Because of this, I think Obama lacks the wherewithal to understand how this legislation could possibly hurt technical people that are not malicious. There's a collective fear in government of anyone technical.
I think that if Obama had a working understanding of computers, he wouldn't be pushing for this. He'd understand how it could hurt the good guys.
Maybe I'm naive though.
I think I fall on the opposite end of spectrum from you, as I tend to think that 99% percent of the politicians holding office at high levels don't actually care about their constituents. I have yet to see any evidence that contradicts my current belief.
If Obama doesn't fully understand what he signed, that would be even more frightening than him not caring.