Some are obvious, like Binyamin which is the Arabic for Benjamin, and we know how hated Israel's Benjamin Netanyahu is in the pro-Palestinian world. Others are prohibited because they:
contradict the culture or religion of the Kingdom, according to reports by local media[snip]
... they are considered “blasphemous,” non-Arabic or non-Islamic, or contradictory to the kingdom’s culture or religion..."
The ban was also allegedly justified by the ministry because some of the names were deemed foreign or "inappropriate".
Other sets of forbidden names include those with royal connotations, such as Sumuw (highness), Malek (king) and Malika (queen).Here's all 50 banned names.
Binyamin (Arabic for Benjamin)
Al Mamlaka (the kingdom)
Rama (Hindu god)
Basmala (utterance of the name of God)
Jibreel (angel Gabriel)
Nabiyya (female prophet)
Pretty darn wacky.
Source: Independent UK