Transcribing a signature is never legal. Although transcribing the printed name, address, zip code, county, date, etc. can be legal in certain jurisdictions.
Transcribing is done by the circulator to improve efficieny. A lot of signers don't want to stand and sign every petition, especially if there are 10. So the signer can sign and write all of their information in on the first petitions. Then, on the remaining petitions, they only have to sign their name.
It is also legal in certain jurisdictions to hire sometime to transcribe for the circulator. In this case, high school students with exceptional handwriting often work for sometimes $10/hour or sometimes $.10 per signature. (They don't sign the actual signature. They only print in the missing information.)
States where transcribing is legal:
There are also a number of states that do not say whether or not it is legal to transcribe. Petition drive companies, in this case, decide not to allow circulators to transcribe just to be on the safe side.