Kalinka, you should consider the following:

1. Iran has no ballistic missile designs that are capable of hitting the US territory (in fact, Iran can't hit even EU with its current missiles).
2. It would be years or even decades before Iran could create an intercontinental ballistic missile.
3. It would take years to enrich enough Uranium to create at least one device.
4. Even if we imagine that Iran has the technology, funds and facility to create such weapons, the most probable target would be Israel rather than USA.

5. From the other hand, North Korea has succeeded in testing its ballistic missiles which experts say could soon be powerful enough to hit targets in North America. And it succeeded in testing a nuclear device also.

There are several questions:

1. Why the US don't make haste in building an ABM installation near Korea considering more real a threat from there?
2. Why wouldn't US build an ABM facility in Turkey instead? Turkey is the most advantageous region for that purpose - you could hit enemy missiles on the acceleration phase when they are vulnerable the most. And Turkey is the NATO member - you don't need tons of paperwork as it has been with Poland and Czech.
3. Russia has the radar in Azerbaijan that covers the whole Iran territory. Why didn't US use it?

I'll tell you why.
Building radar facilities in Poland allows NATO cover the whole European part of Russian airspace. US can claim we're friends but this doesn't demean the fact that this ABM facility gives an early warning about ballistic missile launches. So this facility provides an advantage in case of hypotetical nuclear exchange between USA and Russia. Thus many russian nuclear silos risk being destroyed even before missiles are launched (some older types of missiles require 20-30 minutes to fuel up while SLBM launched from the sub in Arctic can hit the launch site in 5-10 minutes).
The nuclear parity has been guaranteeing peace for the last 60 years, now USA is trying to shift the balance. Of course its 10 interceptors won't hold the whole Russian arsenal, but who said that there will always be only 10 of them?

There's more:
If USA is concerned about nuclear program of Iran it could invade it and bomb it to dust as it did with many other countries, but no - they mantain that they're building the site for defending Europe from some hypotetical nuclear threat in future while they could spy on Russian airspace even today with this radar. I repeatt - it will be years or even decades before Iran is capable of delivering a nuclear payload onto USA (if this ever happens).