Most surrogates are not selfless--- they get paid for what they do...in the US often $60,000. Some jurisdictions do require that women choosing surrogacy have to do it altruistically, receiving only reimbursement for medical expenses. This causes a shortage of surrogates compared to market demand. People looking for surrogates often then travel to jurisdictions where the laws are different to obtain them (I know people personally who have done this). The US is not the only destination, and there are less protections in other markets than in the US. Surrogacy is certainly not prostitution, but poorly regulated markets exist and pose similar issues.