I believe that distributing the Android adb executable is perfectly legal. The SDK license agreement for android sdk at http://developer.android.com/sdk/terms.html has the following clause:
3.5 Use, reproduction and distribution of components of the SDK licensed under an open source software license are governed solely by
the terms of that open source software license and not this License
Agreement.
And since the adb client, which is located under the android tree at platform/system/core/adb is licensed under the http://www.apache.org/licenses/LICENSE-2.0, it should be perfectly legal.
But I'm not a lawyer, so don't take my word for granted and look for professional advice.
