More like they made an engineering decision that when moving the antenna to the outside of the device, the improved overall reception outweighed the increased signal attenuation from touching the device in one specific place. All reviews I have read have agreed that antenna performance is improved overall. Apple simply misjudged which issue would be a bigger concern to the average consumer.
This will likely also be a tradeoff of putting the phone in a case. The attenuation issue will be reduced, but so will overall antenna performance.
The bottom line is, with current technology, antenna performance is limited by whatever the antenna has to transmit through, whether it is the casing or whatever surfaces an external antenna comes in contact with. As strange as it may sound, SJ's suggestion to hold the device in a different way may be the best advice to get the best possible reception from your phone.
One final note: it may be possible to boost the power sent to the antenna when the signal attenuates. This may allow Apple to address this issue with a firmware update, but this would entail a tradeoff with battery life. So, as you can see, engineering decisions are never just "put in a better antenna"; there is always a price to pay in terms of some other attribute of the device, and you must decide what engineering cost you are prepared to pay for the desired result.