Looks like it is legal, against my intuition on the matter.

From http://vpizza.org/~jmeehan/balloon/:

Part 101 of the FAA Regulations covers balloons, kites and rockets. The first section of Part 101 spells out exactly which kinds of devices the rest of Part 101 applies to. Regarding "unmanned free balloons," Part 101 applies if the balloon:

  (i) Carries a payload package that weighs more than four pounds and has
  a weight/size ratio of more than three ounces per square inch on any 
  surface of the package, determined by dividing the total weight in ounces 
  of the payload package by the area in square inches of its smallest surface;

  (ii) Carries a payload package that weighs more than six pounds;

  (iii) Carries a payload, of two or more packages, that weighs more than
  12 pounds; or

  (iv) Uses a rope or other device for suspension of the payload that 
  requires an impact force of more than 50 pounds to separate the 
  suspended payload from the balloon.