the data from the device isn't being transmitted "over the web". The data stays secure on the device. Once the device opened the defense has the right to also examine the phone and determine if "evidence was planted".
Granting access to outside computers means you can't disprove that outside meddling hasn't happened. I would expect a certified forensics lab to maintain a strict firewall for all electronic devices.
I guess the prospect of outside meddling will seem sufficiently unlikely that juries will not view it as much of a hindrance. After all, even with a chain of custody, there are innumerable opportunities for evidence to be planted. You just have to get one or two people who are willing to lie about things. In the US, the standard is "reasonable doubt," and the faint chance that evidence is tampered with during this unlock process will not likely trip the "reasonable" part of that standard, even if it does add some non-zero amount of doubt.
It's a black box that you don't know quite how it works, and you are required to give a third party access to it at all times. I don't know how you can state with certainty that nothing else on the phone was changed.