The original BSD license did have a credit clause which led to considerable trouble and controversy, so they don't do that any more. IANAL, and this is extremely bad manners on someone's part, but no laws were broken.
“Courts have distinguished between copyright infringement and theft. For instance, the United States Supreme Court held in Dowling v. United States (1985) that bootleg phonorecords did not constitute stolen property. ” - https://en.wikipedia.org/wiki/Copyright_infringement#"Theft"
The grandparent poster is correct -- copyright infringement is not theft and the two ought not be conflated. https://www.gnu.org/philosophy/words-to-avoid.html#Theft explains why: "Under the US legal system, copyright infringement is not theft. Laws about theft are not applicable to copyright infringement (http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vo...). The supporters of repressive copyright are making an appeal to authority—and misrepresenting what authority says.".
The GNU Project also recommends pointing people who misuse the word "theft" in this way to https://www.theguardian.com/books/2013/may/04/harper-lee-kil... -- "which shows what can properly be described as 'copyright theft.'".
Please don't join people in getting this wrong. Even some of the most hostile abusers of that conflation have recanted (the MPAA was denied use of that conflation per https://torrentfreak.com/mpaa-banned-from-using-piracy-and-t... and the MPAA's Chris Dodd admitted that conflation was wrong according to https://torrentfreak.com/mpaa-piracy-is-not-theft-after-all-...).
The copyright holder in this case has options including suing, but calling what happened as "theft" is not an option and is unlikely to get anywhere with a judge.
Here are the respective codebases as of Christmas Day 2015, 21 days into the lifetime of one and 11 days into the lifetime of the other. They do not appear to match that description.
* https://github.com/lerna/lerna/tree/9aabe1664399d5f233a89d37...
* https://github.com/Microsoft/web-build-tools/tree/c4bb2127e6...