Is it ethical to have manditory questions in an online CrowdSourced survey with paid particpants?

Our School Research Ethics Committee requires that when designing an online survey that each question is optional for the participants. These rules were devised before the era of CrowdSourceing when researchers had to rely on the goodwill of others to gather the necessary responses.

However as I am paying each participant to complete this survey (using Amazon Mechanical Turk) do you think I have a case to argue that I should be able to make some, or even all, the questions mandatory?

I have not approached the Research Ethics Committee about this yet, I just wanted to see what the norm is in other countries and institutions.

Thanks