It's against the law to drink beer while standing up

That's true in Amsterdam.  Confirmation from Radio Netherlands Worldwide:

People on Amsterdam's terraces will soon be allowed to drink beer standing again, a municipal spokeswoman announced on Saturday...

The ban was originally instituted to prevent excessive noise levels. “Terraces offer limited seating. If people are allowed to drink standing, there’s an increased risk of unacceptable noise levels.” The municipality, however, rather than fining people for drinking beer standing, will from now on focus on combating high noise levels themselves, and will impose fines on anyone breaching the relevant municipal rules, the spokesperson explained. 

There's no indication whether it is also the law in Amersfoort.