Yes, definitely standard practice. At an outstation where an airline only has a small number of flights a day the check-in desk is likely not even staffed outside those times.
Anyway, AA already mentioned above, here's united too
For flights within the U.S., as well as most international flights, baggage will generally not be accepted for check-in more than four hours before a scheduled flight departure, although this policy varies by airport and daily check-in counter hours. United has the right to cancel reservations, deny boarding and refuse the acceptance of checked baggage of any customer who does not meet the applicable check-in time limits, as noted above.
https://www.united.com/ual/en/us/fly...t/process.html
I can't find a first hand link for Delta, but seatguru also reporting as 4 https://seatguru.com/airlines/Delta_Airlines/checkin.php
JetBlue is six
https://www.jetblue.com/help/checking-bags-at-kiosk