Why cruise at 7000' in an A319?
The only reason for your flight to operate at such low altitude is because it is cheaper for them to do so. 


As you said it is due to weather, other route/altitude may not be available. They can cancel the flight but that is likely to be costly. They may have to find accomodation for you and crew until they can put you to the next flight. Sub-optimal flight maybe better than no flight in this case.
They may need that aircraft at that location the next day. If they choose to cancel your flight today, they will have to find a way to have this aircraft or another aircraft to fill in for the required flight the next day. They might as well fly a slightly more expensive flight at lower altitude rather than fly an empty plane to Boston.
It is possible for them to wait for weather to improve but there is an issue of crew working hour. A pilot cannot work more than 8 hours in a day without provision for additional crew. They may approach that limit and the wait for weather to improve might means the airline has to provide another crew for the flight and might mess up crew schedule for the next few days.


So for the airline, in this case, flying lower and slower maybe a better option for them. Hence higher fuel comsumption is better than no flight.