A backdoor bypasses legitimate access mechanisms. Whether it is a backdoor or not depends fully on whether you believe lawful intercept is a legitimate access mechanism. And I think the law is on the side of it being not a backdoor.
It's seems there's a semantic schism on "the point in software where security is weak enough (either purposefully or unkowingly) for 3rd party access by a 3rd party" and "the point in software where security is purposefully weakened for 3rd party access by the legal requirement of a 3rd party" there's definitely a distinction but I generally conflate the two, perhaps incorrectly, under the word backdoor.
If you've got another more appropos word for this purposeful and legal weakining of security for non primary user/provider access I'd love to know it because sadly I feel I'd use it fairly often in the coming years.
I'll also add that ironically those who don't oppose these bills are unwittingly doing the bidding of strategic adversaries as demonstrated quite adequately by the PRC here.
The problem is not whether the backdoor was legally mandated or not, and whether legal authorities are misusing them or not, the problem is that it exists. And the existence by itself is enough to let someone ignore any legal mandates and view the comms.