Jazz theory question

Here's something I've been trying to understand... take a standard jazz progresion in Aminor. It would look like this: Bm7(b5)-E7(b9)-Am7

My question is this: Why would the V chord (E) be major? A minor contains Em, not Emajor- unless of course you're talking A harmonic minor.
I'm sure this is brain candy to some of you but I've been struggling to understand why this progression would have an E7 and not an Em9!

oops, at the end I meant Em7, not Em9...

James- you answered your own question, the dominant 5 chord is borrowed from the harmonic minor scale. When in a minor key it is common to utilize all 3 forms of minor together. For example, in that example you gave the 5 chord is taken from the harmonic minor but the 1 chord is from the natural minor.
The reason for the dominant 5 vrs the minor 5 is that the dom. has more pull to resolve to the 1 chord (E7 contains G#, which is the leading tone to A)
I can't think of any examples where the natural minor 5 chord is used in a minor 2 5 1 prog., it's always approached as a dominant.

It is common to use dominant chords in place of the min7 simply because the dominant has more possibillities. The chord can be altere in many more ways, and more interesting scales can be used over it, as well as iot having more pull to the tonic.

Thanks for the response guys...
I never considered before this that more then one form of minor might be used in a progression, but here's the proof. Theory is great stuff...

oh, btw... an example of a form of music in which the fifth remains minor and not major would be celtic. A celtic progression might go, Em - D - Bm - Em - Bm - Bm - Em

The five always being dominant was in reference to a 2 5 1 prog, you always see B-7b5 to E7 to A-7, never B-7b5 to E-7 To A-7 (or at least I never have, still searching)