That is not true, you can use as many as you want on one account. I've used like 3 or 4 at this point ! some sellers have requirements some don't just read carefully
Do you even know what you are talking about? If you've been able to do what you say you did, you are obviously not doing it legally. According to Bing Ads: Limit: one ad credit per new advertiser.
Its actually legal ! You can only use that ad credit that bing provides through there sign up once that you can only once. But other market places offer bing ads fresh bing ads again fresh meaning they haven't been used and refilled or any illegal thing your saying! Some countries have limits that you have to be aware, but where I live you can do 3 to 4 a month i've done 2 at this point doesnt effect my account nor is it illegal so I know what I am saying
actually you can use multiple coupons in bing ads. I know one guy he $1000 worth vouchers every 3 months in his bing account. But yes you cant do this in Google.