[Rich in Vancouver]

Having worked in the trade I can see their point, particularly if they have to warranty the job. If the customer buys the cheapest parts off the net and they fail the shop stands to lose money in labour time replacing them. Margins tend to be quite small these days and they may not want to take the chance.
Customers are all smiles and friendly when they are talking you into doing work you are not comfortable with but as soon as things go wrong they change their attitude.
Most of us who do our own work have had issues with substandard parts and we aren't paying a wage, or overhead on a shop. Even when I help out mates with a job I worry about getting a phone call if their car breaks down after.