As it's probability^1.5, it makes the probability lower, and not higher, i.e., x^1.5 < x if 0 <= x <=1.
I think this might be a tweak due to data scarcity. To compute p(e), there's tons of data to build a model. On the other hand, p(e|f) requires for you to get a lot of parallel corpus (texts that's written both in English and Foreign language) which is not easily doable.
As a result, p(e)^1.5 * p(e|f), lowers p(e) as it's supposed to be too high compared to p(e|f), imho.