if i want a copy of the film i've purchased to play in a portable DVD player i have to buy another copy from HMV, Amazon or somewhere like that
Yes, that is right. The content owners do not allow Apple to offer burning such content to video DVD as a feature.
i cant load any DVD's i already own onto my ipad
Some DVDs and Blu-Ray movies come with a "digital copy" that allows you to download a copy of the movie for use on computers and portable devices. You might see if any of the DVDs you have come with such a feature. Walmart/Vudu also has a service that basicially gives you a digital copy of a DVD you already own. See:
http://www.vudu.com/disc_to_digital.html
Otherwise, you are correct; the laws in the US and several other countries make it illegal to break the encryption on the DVD (or Blu-Ray) which is necessary to convert to another format. Stupid, I know, but that's the law.
Regards.