The following translations have rendered Matthew 6:10
ως εν ουρανω και επι της γης [Elzevir 1624]
as follows :-
Tyndale : as well in earth as it is in heaven
J N Darby : as in heaven, so upon the earth
YLT: as in heaven, also on the earth
KJV: as it is in heaven
The KJV is, as you point out, ambiguous, but the other three are not and those three translate the phrase in the manner of 'like the way it is done in heaven' not 'because it is done in heaven'.
Young's Ananlytical Concordance lists the following Greek words as those which have been translated 'because' in the KJV :
δια 29 times; εκ twice; εν 3 times; επι once; προς once; χαριν twice.
ως has never been translated 'because' in the KJV.
It has been translated :- as 344, about 14, as it were 20, how 19, when 41, while 4, as soon as 7, even as 6, that 5 and sundry others. But never 'because'.