From my experience over some 25 years of being in various bands, making ground rules is a real no no, having said that you must all be signing from the same hymn sheet. You need to ask yourself what you want and where you see the band going. It sounds to me like the band or at least one member see's you as a weak link, the fact this rule was cooked up in your absence and without your agreement seems a little convienent and harsh.
Ask yourself what you want from a band and if the current band do not meet your goals then leave and start your own from scratch and ensure your new band mates have similar goals and targets to you.