I am not familiar with how the warranty works. With a car, the dealer is reimbursed for any warranty work. The work can be done at any dealership. If that's the case with the lawnmower, why didn't you do it?
The warranty work you are talking about is on newer vehicles. Warranty work for a newer car under warranty cannot be performed at any shop. The shop has to be set up to perform that make/model etc. car warranty, so they can get paid, from the manufacturer ie: Ford, Chevy.
I do NO warranty work on newer outdoor power equipment at all. Example: There is a three year warranty from Kohler (Relhko), and the shop does the warranty work, and gets reimbursed from Relhko, not the customer. I think in this thread, some people are not clear on warranty.
As far as do it yourself (DIY) that work under the table, that ain’t me. I have liability insurance, pay all state, federal, and sales taxes. I have a business license and initially went through local planning and zoning. That whole nightmare of a process took 6 months.
A large part of the reason I am so busy is because I do great work, have nothing but integrity, and the majority of my business is based on repeat business and referrals.
I don’t worry about what other shops do. I am aware of it often from customer feedback. Everyone does business a little differently. I just concern myself with my business and how I take care of the customer.