In March, terrorist Khalid el-Bakraoui blew himself up inside Brussels’ Maelbeek subway station, and 16 people died. Several years earlier, Bakraoui had served time in a Belgian prison for carjacking and armed robbery. When he was released from prison in early 2014, Belgium’s welfare system gave him $28,000 in various benefits, such as unemployment and medical insurance.

Bakraoui is one example in a remarkable story this week by Wall Street Journal reporters Mark Maremont and Valentina Pop, who described how Europe’s generous...