America has done quite a lot for Greece.
The moral support the United States has given Greece at a diplomatic level is beyond question and they have been supportive of all the bail outs and even a hair cut to the Bank owned liabilities.
And, it's not America's responsibility. It's not as if they will pay Greece's debt but they do send a lot of aid for the Syrian refugees and they also give Greece some military aid to protect its borders.
And the POTUS visit is a huge honour since America chose to visit Greece which is another country which shares the same democratic principals, western values, the rule of law and to reiterate the shared interests and cultural links between the 2 countries which are extensive.
It has done far more than the EU and Merkel and Obama promised Tsipras that he will be sending a strong message to Merkal for a haircut on the debt.
Greece is also a critical ally to America, an important NATO member and Greece also hosts the USN in its ports and there are a number of bases in Greece as well.
Israel is another important ally to Greece as you can see for yourself when the IAF dropped in to say HELLO!