Name collisions are going to happen. There are a limited number of cool names. If it's not causing any real problems, then I don't see the big issue.
That said, if I accidentally caused some guy to get a bunch of tweets not meant for him, I'd probably apologize about it privately (obviously it wasn't intentional). I definitely would try to talk to the guy before trying to get his twitter account suspended.
Would you be annoyed/angry if someone took your twitter handle and primary domain name and started using it as their own? If though you are very active and controlled that name for a number of years?
Almost everyone would be angry (or very annoyed). There are a limitless number of names out there.
Here is why it is a dick move.
1) They almost for sure knew about him and his name.
2) They didn't contact him or offer to buy out his domain/handle.
3) They went ahead with it anyways probably because he hadn't trademarked it.
Look it's not the end of the world, it's just a pretty shitty thing to do to a person.
If someone used the same name that I am using for an internet nickname for myself (a person) for their company, I would essentially not care. At worst I'd be annoyed at an increased number of "@" mentions that comes from there being namespace collision.
And, frankly, this happens all the time. There is a ton of name collision out there happening as names which are sufficiently unique for a local context get moved online, which is a global context. Generally, when people find out there's been some name confusion, they are mildly annoyed by the inconvenience of people "@" messaging the wrong account, but not indignant that someone dared use the same name as them. Mostly when I see this happen I see people getting upset about the people "@" messaging them not checking the name more carefully, not at the company for picking the name. That is a pretty strong indicator, to me, that most people wouldn't consider this a "dick move".
Asshole is probably too harsh a word, but considering that "futurestack" was already taken as a domain and as a twitter account and both were in active use and related to technology, naming your conference "futurestack" seems a bit lame. Perhaps they could have done more to communicate what the official domain / twitter accounts were. (i.e. make them more prominent)
This conference stole his identity. He isn't claiming he has a right to the name.
What the conference did was legal, but still not right.
All too typical of people trying to build themselves up in status.
They knew about this guy and they then they checked to see if he trademarked it. They knew they were legally okay, so they went ahead with it.
Legal move, but a dick move.