I have been with my company for 10 years. now they are offering me an "entry-level" promotion. WTF?

I have been with my current employer for 10 years. For the last 5 years, I have been the warehouse lead. It is an hourly position and with overtime I make approximately 48,000.00 per year. I also have a four -year degree from Washington State University. My company has just offered me a promotion that I have been waiting three years to be offered: Warehouse Supervisor...it is the next logical step after Lead. It is a salaried position. I was excited until I was told that the salary being offered me was $40,000.00 per year, with potential of a yearly 6% bonus (I've heard that after taxes the bonus equals about $1,500.00). I am being told that there is no negotiation...this is an "entry-level position because you have no supervisory experience." Firstly, I have 5 years of supervisory experience as a lead with this company, and 12 years as a superintendent supervising multiple work crews at a previous company. I submitted an updated resume outlining all of this information when I applied for the in-house position. Since when does a promotion require you to take an $8,000. pay cut? In any case, my question is this: Is it even possible for someone who has worked for a company for 10 years to be offered an "entry-level" position? Isn't that an oxymoron?