They may have recieved bad/poor service, BUT...
...there are several things you seem NOT to be familiar with.
#1 Metal and braze joints do have pores. Whether they are large enough to cause problems is a different point.
#2 Depending on the size of leak, it 'may or may not' be cost effective to fix and repair the leak. (although, I do prefer to find/repair them)
#3 '25% low...' That can be an easy... What is the total system charge? Then if they weighed the amount (like they should have). They could easily figure out what percentage that is of the total charge. They would first have to make sure 'everything else' with the system is correct.
#4 Line set lengths normally change the charge by oz NOT lbs.
#5 'Low on charge causing problems...' Would all depend on conditions and run times.
#6 'Low on charge cause slugging...' No possible way. Do you know what slugging means???
#7 'I told them it a system is done right it should never need freon to be added...' I think this is the only thing you should assume to be correct in your post! :D
(although a couple of oz in a system in 18-20 years can be achieved due to hose connecting/disconnecting)