I suspect the people kicking up a fuss about news coverage know the reasons full well, but don't let that stop a good moan.
Bombings on US soil are HIGHLY unusual and throw up the possibility of either domestic 'white' terrorism (ie Timothy McVeigh), domestic Islamic terrorism, or overseas terrorism (likely to be Islamic, but rumours of North Korea!). Or something new. An unusual high profile event will always get more coverage than the commonplace.
We (the typical Westernised Brit) relate much more to the US. We have closer ties to them, we see their culture on our TVs almost 24/7. Most of us know many Americans personally, indeed some of us work for American companies and go there regularly. It's the next thing to a bombing on British soil.
There is actual TV footage of one of the blasts, and huge volumes of footage of the aftermath. The scale of media focus in the US is on another level entirely than anywhere else on the planet and guarantees blanket coverage. All news networks want dramatic footage and the Boston bombings have delivered in spades. You even see a runner collapse - what more could they ask!?
A bombing on US soil will trigger a much more high profile reaction. From Obama making a speech (and presumably a visit to the site) to the hunt for the perpetrators, the media will be on hand every moment and our news networks will lap it up - again, an unusual event demands increased focus. If domestic, what measures will be taken? If overseas, what will the reaction be and how far will that reaction go?
Bombings in Iraq are commonplace. As cold-hearted as it is, that's a fact. Footage of Iraqis standing around debris is nothing new. Almost guaranteed to be a Sunni/Shiite attack. It doesn't make it right but there it is.
If you don't like it, complain. But guess what, we've all watched it and the news networks know we'll watch it.