If you ask them why they say its because it'll often dissolve the pipes before the blockage. Most of the trades people I talk to aren't looking to maximize their work. I've had them turn down jobs for being too gross and I don't blame them.

They'll generally use a snake/auger. Home Depot sells a basic one for $15. The beefier, further reaching ones cost more. Or, for smaller jobs, there are single-use plastic snakes.