You must have missed the part where it said - there are only 24 Muslims on Quantico base. So a Mosque is being built for this tiny group?
Yes, a Mosque was built so that Marines, putting their lives on the line every day to provide you your Constitutional rights, might actually get to enjoy freedom of religion like you.
Small group, small Mosque. Besides which, 24 is bigger than a number of small town congregations in the US.