Young Paul.
No design flaw. The equipment would only need to operate on 6-8 spot frequencies, hence would have a relativly small range to be tested against the A/C kit for interferance. The alternative is SLF bringing different makes/models of cellphones operating over a very wide frequency range - practically impossible to prove no interferance is caused to A/C kit. You could always harden the A/C kit to MIL standards - very expensive, but would stop interferance to A/C kit. Modern civilian A/C are much better protected against EMI than old, but not to MIL standards.