Well, sure, they have the RIGHT, but I don't cross those kinds of lines if I can avoid it.
It's a simple matter of respect.
I love the BLADE character and I thought BLADE vs. AGENT X would be awesome. So I asked Marv Wolfman and he was nice about it, said he couldn't stop it, but it clearly made him uncomfortable, a friend of his using Blade with all the enmity between him and Marvel over that character. So I didn't.
Later, I wanted to use Black Lightning in BOP, another favorite of mine, and I asked Tony Isabella, and I THOUGHT he was saying something similar to Marv, so I didn't use BL. Later we talked about it and I'd misunderstood him, he was happy to have me use BL respectfully and appreciated being asked.
I've asked previous Wonder Woman creators about their additions many times, and almost universally, they've all been incredible and given their total permission and blessing. The one exception was Veronica Cale, whom Greg Rucka had reservations about, I think MOSTLY that she was going to be used elsewhere. But he's been great about everything else from his run I've asked about, including suggesting that I use Ferdinand. That's why he got a 'thank you' credit in the book recently.
But yeah, I don't cross those lines. If John hadn't said okay, it wouldn't have happened.