If the businesses were worried about cash flow, I think the 75% haircut would be the bigger issue. Also, they will likely be pre-paid for a large percentage of their Groupon patrons (I have many I haven't used from months ago) and not everyone ends up using their coupon.

Businesses flock to Groupon because its popular and puts them in the spotlight for a day to tens or hundreds of thousands of local people on their mailing list and twitter followers.