because the major Kashrus organizations say you can.
heres a statement from OU:
a little wishy-washy.
a more decisive statement from cRc:
“All unflavored beers, domestic and imported, with no additives listed on the ingredient label are acceptable, even without a Kosher certification. This applies to both USA and imported beers, including non-alcoholic and dark beers.”
flavored beers need a heksher