If you can laser weld powder into solid mettle why cant you laser weld two pieces of solid mettle together even if you need a little powder to help?

Frame 1.4Kg!

Slowburn,
Direct metal laser sintering fuses a thin layer of powder at a time. Each layer is in the order of microns thick. It doesn't produce enough energy to penetrate deep into a part, which would also risk heat distortion. If you want to weld titanium parts, you'll have to rely on good, old fashioned TIG welding, which is too risky with a frame like this that's been lightened as much as possible.

I suggest the Author reads about laser sintering titanium and the costs. time and size all need to be taken into consideration. That's probably a good £40k frame they've built there - if not more.
http://www.core77.com/blog/exclusive/exclusive_photos_making-of_the_queens_baton_for_the_xx_commonwealth_designed_by_4c_design_ltd_25735.asp

Be careful on you phrasing of cause and effect. Lead is also denser than aluminum but it wouldn't make a great bike frame. Titanium is stronger than aluminum might be a better phrase.

The other thing to consider is bed size on the 3D printer. To do an entire frame in one piece would need a bed at least one yard or meter square. The picture looks like several parts were made concurrenlty on a bed that might be 8"x8". Still, quite the accomplishment. And like the 1911 recently done elsewhere, quite the price tag I bet.

Given the size of the bed that would be needed for printing and the inefficiencies that would go along with printing a whole frame, pieces are the key. And even though I have welded Ti quite a bit, the socketed route is the way to go. You only have to glue the pieces together. Where as the TIG welding of them would require a special jig, argon purging of the frame, specialized welding cups and skilled labor. And maybe the first couple bikes could cost in the 10's of thousands to help recoup R&D, but with an efficient crew that would keep the flow of parts flowing through their million dollar printer, I could see the prices dropping to a somewhat reasonable level.

Although the article says there is no wast, It appears to me that there is some wasted material. The base and the "sprue" sections in the photo appear to be titanium which would be separated and become scrap.
As far as joining the pieces, I believe titanium can be brazed, perhaps with a titanium/silver alloy in a vacuum furnace. That would result in an extremely durable final assembly.

Brainfarth has it right: creating the connecting pieces is where 3d printing works best. Once you have those joints it's easy to finish the bike with mass produced tubes.

Be interesting to see case studies in extreme environments. Asphalt areas in the southwest U.S.A. apparently get around 130 degrees Fahrenheit. The far north gets very cold too. Not to mention stress testing in extreme freestyle, downhill mtn biking, ect.... A very competent testing evaluation should be very interesting.