A dictionary should function as an aid to translation. For an entry to be valid, it should "work" in both directions.
It may seem all very well for bad English grammar to be translated into good German; but students should not be misled into believing that it is acceptable to translate good German into incorrect English. Somebody has to make this point. If LEO will not do this, then I will.
Which is why, as CM2DD pointed out, the expression "for free", which does exist and is used by native speakers of the English language, should be tagged "informal" or "colloquial" and paired with equally informal/colloquial German expressions such as "für lau" rather than with "kostenlos", which is the wrong register. But remain in the dictionary it should.
As to commas, I use them in the way I was taught - before conjunctions, wrapped around relative clauses, where a slight pause is intended, or where there is a slight change in direction.
I'd recommend looking up "defining and non-defining relative clauses", at least I believe that is the correct name. Or, possibly, "Eats, shoots and leaves" by Lynne Truss (sp.?), a book I have heard of but not read myself. Maybe you'll rethink some of your commas. Only very few English native speakers would place a comma in this sentence, for example:
Are Germans the only ones in the world, who are allowed to be fussy about the grammar of their national language?