Forcing a page to open in a new window means that your website is not "accessible" and contravenes the WCAG (Web Content Accessibility Guidelines). So what you might say .... well a blind person visiting your website using screen reader software would not be able to navigate your website.
There have been cases of web developers/companies being sued because their lack of website accessibility was seen as discrimination. I'm not sure what it is like in the USA, but down-under we have legislation, Disability Discrimination Act 1992, that covers this very point, ..... a person (discriminator) discriminates against another person with a disability (aggrieved person) if the discriminator treats the aggrieved person less favourably because of the fact that the aggrieved person is accompanied by a reader .....