Two points with regards to that: 1) Parallel construction has nothing to do with the iPhone encryption issue. 2) People tend to overlook a few key points from the original Reuters article[1] that introduced the concept of parallel construction:
(emphasis mine)
"...Today, the SOD offers at least three services to federal, state and local law enforcement agents: coordinating international investigations such as the Bout case; distributing tips from overseas NSA intercepts, informants, foreign law enforcement partners and domestic wiretaps; and circulating tips from a massive database known as DICE. ...
...Wiretap tips forwarded by the SOD usually come from foreign governments, U.S. intelligence agencies or court-authorized domestic phone recordings. Because warrantless eavesdropping on Americans is illegal, tips from intelligence agencies are generally not forwarded to the SOD until a caller's citizenship can be verified, according to one senior law enforcement official and one former U.S. military intelligence analyst."
Nonsense - parallel construction (and any other widespread use of the capabilities the NSA/etc is providing to other "their customers" (DEA/FBI/various-local-PD)) is likely the primary reason for this attack on Apple and encryption on the iPhone. The data covered by that kind of encryption would likely be of very limited use for national security; the NSA gets most of the data at the backbone switches anyway, and encryption doesn't do anything to prevent relationship mapping from logs of routing metadata. Also, any kind of targeted investigation could simply bypass encryption by installing custom firmware. Several of the tools in the TAO catalog were based on that style of attack.
What would be lost with local iPhone encryption keys is the ability to gather large amounts of data by strong-arming Apple (Prism, possibly). Note that most of the people freaking out over Apple's changes are not NSA. It is law enforcement who is fearing losing their access; the same law enforcement that would be using parallel construction to actually use the data that logically they didn't have a warrant to search and seize. (if they did have a warrant, they can bypass the encryption with various other ways, which apparently includes compelling passwords)
As for the Reuters article, I linked to a specific document that was a follow-up to that Reuters article, which had very little to do with foreign governments, and a lot to do with protecting access to the surveillance infrastructure. If you want the TL;DR version (understandable; it's 300 pages of slides and forms), [1] is a decent overview though it lacks some of the relevant details.
This issue has absolutely nothing to do with the NSA. As you point out yourself, this applies only to data stored locally on the phone. When was the last time the NSA had physical access to your phone?
Your argument seems to be that law enforcement wants to keep the phones unencrypted so that they can seize them with a warrant, hand them over to the NSA, and then the NSA can hand the data back to the police using "parallel construction" in order for the police to hide where the data came from (i.e.: acquired lawfully by the police with a warrant)
(emphasis mine)
"...Today, the SOD offers at least three services to federal, state and local law enforcement agents: coordinating international investigations such as the Bout case; distributing tips from overseas NSA intercepts, informants, foreign law enforcement partners and domestic wiretaps; and circulating tips from a massive database known as DICE. ...
...Wiretap tips forwarded by the SOD usually come from foreign governments, U.S. intelligence agencies or court-authorized domestic phone recordings. Because warrantless eavesdropping on Americans is illegal, tips from intelligence agencies are generally not forwarded to the SOD until a caller's citizenship can be verified, according to one senior law enforcement official and one former U.S. military intelligence analyst."
[1] http://www.reuters.com/article/2013/08/05/us-dea-sod-idUSBRE....