Why am I being asked to show a financial POA when I am the spouse paying the bills?

Thanks for the laugh, yes the checks have been sent back twice, same issue. This time I may have to send a form for them to sign saying they refuse payment!

Because they are ignorant.

Send the checks back...send a note. Such as "duh..my name and my HUSBANDS name are on this JOINT account...why do you think this is irregular?"

There are a lot of ignorant people around. Seems like more every day

Good Lord! Makes you want to scream at these people "What the he!! is your problem? I'm trying to GIVE you money!"
Morons.

I'm guessing they want proof that it was okay with your husband that you pay his bills. Which is just nutz as doesn't it usually work that way in marriage?  After all - wouldn't it be you who would be held responsible for an unpaid bill?  And like the other reply says - why does it really matter as long as the freakin' bill gets paid?

Just be prepared for the next idiot move on their behalf - when their accounting department sends you a letter threatening to start Collections for the unpaid bill. 
Morons!

Speaking of the expanding ignorant population, my sis (I am DPOA) needed a special Ultra Sound test at local OP x-ray office. She is 78 and does not drive(found her nearly homeless in 2016.) Office wouldn't schedule appt. without photo ID, which she doesn't have. I have her ins. card, SS card, letters addressed to her from ins. company, monthly statements from her Assisted Living facility, etc. Their insistence with "no photo...no U.S." and the urgent need for test prompted me to say "I will pay for it." Sorry, we can't take it without her photo ID....and only our billing dept. accepts payments."
Laughter remains the best medicine so had to toss this into the mix. And yes she got the test and ins. was billed. I am an NP.

One that I liked was the folks on the phone who would ask me to put my husband on the phone. They would ask, "Is it OK for Jeanne to speak to us about your account?" and he would, of course, say Yes.

I asked the guy once, how do you know that wasn't my brother, or the UPS delivery guy, or my neighbor who stopped in for coffee? He laughed. "I have no idea who that was, ma'am. I just know I can check the right box on this form now." Some of this so-called HIIPA stuff is strictly CYA on their part.

I have no idea why they would do this. Who cares who is paying the bills as long as they are paid. The only time I have had to prove POA to the bank, was when I wanted to move money from CDs (that I was not joint owner of) to Moms checking acct. Even though I am jointly on her acct I do sign with POA but don't think I have to. I agree, your working with an idiot. Now, if u weren't joint owner and u signed as POA, then they could question the signature. This may be the problem. Someone isn't thinking out of the box.

I also got a kick out of when they'd ask to speak to my mother.

In the time it took from me telling my mom who I was calling and why to the point they asked to speak to her - my mom would forget the whole situation. This lead to a good five minutes of "Who's on First" with my mom yelling every other sentence "What? I can't hear you. What are you talking about?  You're mumbling!" I did have to learn to stand right next to mom and be quick because she often said to them next "I don't know why you called me but if you can't annunciate I'm not speaking to you!" and would hang up. Then I had to start all over again - from the very start - "Mom, we're gonna call Comcast ..."

Speaking of phone calls. I had a woman from social security call to talk. She could not talk to me. Insisted on talking with Mom

I explained to her...Mom has profound disphasia along with some dementia. It was likely she could not communicate at all...but, if she could...it is important to skip the small talk and just ask her straight out.

Ok....so, I put Mom on the phone. Idiot woman from SS says to her first thing..."how are you this morning?" (Ah geez, I am thinking). So Mom tells her "a bah dah do dee bo gaba gaba". It was all I could do to keep from busting out loud laughing.
Then she asked Mom her birthday. To this Mom responded "16 47 64"

That is where their conversation ended.

Geez, some people just do not listen. I told her this was what she could expect.

But, it was the best laugh I had that week

My husband is deaf. When he gets a call and they won't talk to me, I tell them he will not hear them even ask the question if it's OK for them it's OK to talk to me. They have to wait for me to explain to him what the call Is about and to just say it's OK to talk to me.

This stuff makes me crazy, too! Have you asked the facility why they want a PoA? Maybe tell the moron that your attorney says you don't need one for this, and ask for a supervisor and/or a written explanation, so you have proof that you've been trying to pay them when they start sending past-due notices.
Is the care they provide your husband as efficient as their financial management? Yikes!