There are legitimate business reasons to do these things but if you want to avoid the situation, make sure to have it clearly stated before the project begins that you expect to have full rights to the source files. Even better, include it in the contract. Or have it written up clearly and agreed to in an email before the project starts.
And if you ever have a wedding, the same thing applies for your wedding video or wedding photos. Work it out ahead of time that you will actually own the raw video or photo files.
And don't be surprised if the prices that you're quoted go up when you ask for these things. Often, contractors will quote low prices knowing that they will make more money in the long run from followup work. It's not wrong for contractors to do that, but it feels dishonest when it's not clearly explained ahead of time exactly what you're getting when you pay for contract work.