It's not quite that simple. If you have customer data that's already encrypted, then compression won't do much because it looks random. But of course by the time you get to your infrastructure layers, that'll be the case (or you really messed up your security story!). Which means you'd have to compress right at the edge. They might be doing that (which would basically mean it's the customer compressing it before they encrypt it with their keys because AWS has no business seeing the clear text), but then you get to compress each item separately, which might not be very effective for small values.

tl;dr:There's a real efficiency/security/insider risk trade-off here.

Edit: I should disclose that I work for a competitor. Don't intend any astroturfing.