You don't seem to understand how business works. This is their job. If they're self-employed, they are still considered to be a business.
http://www.cra-arc.gc.ca/tx/bsnss/sm/menu-eng.html
They are engaging in business transactions to make a profit, not private transactions.
You don't seem to understand that not all 'businesses' are the same before the law. Incidentally, your link doesn't actually apply to prostitutes in Canada; since while it is legal to sell sexual services, prostitution in Canada is *unregulated*. As such, your argument doesn't apply. However, even in jurisdictions where prostitution IS regulated, your argument still doesn't follow. Different jurisdiction define 'business' in different ways (and even then apply different standards between different types of businesses).
In the Netherlands, prostitution is legal and regulated. A prostitute can either be employed in a wage-labour setting, in which case they must sign a contract (but even so they must still give consent, so they can not be made to have sex with potential clients they don't want to) and *do* become part of a business (again, the business *itself* may not discriminate, but since prostitutes must give consent they can not be denied the right to refuse sex on any grounds. Or, alternatively, they can be self-employed (NOT part of a business, and not themselves a business as such).
Dutch law EXPLICITLY criminalizes the act of forcing a prostitute to have sex. Forcing a prostitute to have sex is defined and understood to include ANY of the following criteria (this list is from a brochure on the subject put out by the dutch government to help identify forced prostitution):
• You have to do work that is different to what you were promised;
• You are aged under 18 and are working as a prostitute;
• You are being forced to work as a prostitute, for example by someone who has
arranged the work for you, or by a so-called lover boy;
• The person you are working for is threatening to report you because you are in the
Netherlands illegally;
• You do not have your own passport or travel documents in your possession;
• You have to hand over the money you earn (some or all of it) to someone else;
• You have to work even when you are ill;
• You have to pay off a large debt to the person you are working for;
• You have no say about where you work or live;
• You have to work in different places and often do not know where you are;
• You are not allowed to go back and forth by yourself between where you live and
where you work;
• You are not allowed to go shopping or buy new clothes by yourself;
• You or your family are being mistreated, blackmailed or threatened;
• You work in unpleasant or poor conditions;
• You are forced to have unsafe sex;
• You are forced to perform specific sexual acts;
•
You are not free to refuse customers;
• You have to work long hours;
• You are only allowed to finish work when you have earned a certain amount of money,
or after you have had a minimum number of clients.