If product x has a backdoor, this backdoor will also be used by bad people. It can also be cracked and there will always be product y without a backdoor. Don't the legislators see this? Or are criminals statistically really that lazy that they will still use mobile phones even though they are compromised.
According to "The Wire" they often even put extra layers of encryption security, anyway. Such behavior would make that legislation even more useless, as it would indeed only target the innocent that don't take extra precautions.
It's the usual short-sighted, first-order effects thinking we see so often in politics. Behaviour X is problematic; target a law at behaviour X; ergo "problem solved". But that's not the way it works. Laws can never directly address behaviour X. Instead, the law creates behaviour Y in the state apparatus; Y may be opposed to X, but it also has effects of its own; and the damage from Y may exceed the damage from X.
It can also be used by good (or at least inquisitive) people.
The back door isn't going to be built into super encrypted military stuff, but a lot of government traffic travels over commercial links, using commercial hardware and software.
Since this government backdoor is going to be leaked or hacked about 3.2 femtoseconds after it's introduced then every internal government memo, every cell phone/sms between a politician and their mistress or lobbyist and every conversation between lawyer and client is going to be on wikileaks.
Season 5 how Marlo arranges meets and pickups was a good example of extra security.
SPOILER ALERT
Marlo (the drug lord) sent picture messages of a clock to arrange meetings. The hour and minute hands corresponded to the letter and number axes of a common Baltimore map and the second hand tells what page. The dealers would look up the coordinates on a map to find out where to meet. The cops broke it because they had (illegal) wiretaps on everyone's phones, and they had illegally allocated lots of extra manpower to tail all of the suspected drug dealers around the clock.
According to "The Wire" they often even put extra layers of encryption security, anyway. Such behavior would make that legislation even more useless, as it would indeed only target the innocent that don't take extra precautions.