You know, because of all the terrorist attacks that have taken place in court houses.
1.) Maybe not, but there are a heck of a lot of terrorism trials that go on at those federal courthouses...and if you're gonna build a new building, you may as well make it as secure as possible.
2.) I'm sure the funding is from the feds and was amped up after 9/11, but the real motivation for more secure federal courthouses and buildings is the Oklahoma City Murrah building bombing (that WAS classified as a terrorist attack). If you see any new federal buildings, you'll notice that a number of them have LOTS of green space around them to protect from any sort of vehicle based bomb attack like OKC. Since those buildings are usually in downtown areas, the feds have to buy the real estate which tends to be quite expensive. Then they have to build everything with 3 foot thick concrete and triple bulletproof windows...because let's face it, there are some real bad people who have their lives decided in those places.
I'm pretty conservative on government spending, but even I look at these buildings as damn near a necessity to keep legal and law enforcement officials as safe as possible.