Is it legal for a balloon to land at a US international airport?
Assuming the winds directed a hot air balloon near a US international
  airport


By this I assume you mean a major airport in class B or C (possibly even D) airspace. An "international" airport is simply one that is an airport of entry and maintains customs officials and processing ability and may or may not be a "large/major" airport. 


  would they face legal trouble/fines


This depends on how the balloon is equipped. As mentioned in the comments you need a transponder and a 2 way radio to enter certain controlled airspaces (usually those surrounding big airports) and if the balloon is properly equipped it would be allowed to enter and utilize public resources such as airports and landing areas. This advisory circular on ballooning would indicate it is legal to enter all airspaces in a balloon.

There are of course some practical problems here such as the fact that as a balloon you can't really divert around an airspace if you are not granted clearance. If there is no safe landing space and the wind is blowing such that your only option is to enter the class B you may be stuck declaring an emergency, or at least getting back on the radio and explain whats going on. Some FAA officials may consider this poor/dangerous flight planning (since you should have looked at the wind prior to departure) and you may end up in some trouble for that.      

From the Balloon Hand Book


  As a general rule, balloons do not freely operate within Class B
  airspace. Equipment requirements are the same as for Class C airspace;
  however, due to air traffic congestion, the balloon pilot requesting
  entry to Class B airspace will likely be denied entry, as ballooning
  operations inside the Class B airspace constitute a potential traffic
  conflict....


But legally you can and the FAA prefers you coordinate before hand,


  Should it become necessary for operational reasons to fly through
  Class B airspace, that flight should be coordinated at least one hour
  prior, as provided for by 14 CFR section
  91.215. It is permissible, and perfectly legal, to operate a balloon under the lateral limits of the Class B airspace.


Same applies for class C, 


  Balloon operations in Class C airspace, while technically feasible,
  are usually not advisable