I'm also curious about this - from what I remember from the relevant bits of history lessons, the early antitrust laws were enacted at least in part to quash the price-fixing of guilds.

Unions have rights to conduct in collective bargaining, but I'm not aware of any similar allowances for guilds of independent contractors to set prices collectively where it would have anticompetitive effects - they're treated just the same as other traders / companies.

I am curious if the difference is having such an organization as an option versus finding people off of the street. In that way, using a guild is one option, but they do not prevent you from using other options.