In general, California gift card law does not allow for gift cards/gift certificates to be sold with expiration dates; post-sale fees are also generally prohibited. However, California gift card law (and federal gift card law) specifically provide an exclusion for promotional gift certificates. California Civil Code Section 1749.5(d)(1).
Thus, if Groupon clearly discloses the expiration dates for both the promotional value and cash value, Groupon does not appear to be breaking any laws. The bottom line is that if you paid $5 for $15 worth of goods, you can always get what you paid for, e.g. $5, even though the $10 promotion value may expire.