Another common issue is, that by posting their content on Behance in your name, their brand is on a platform they can't control. It may be difficult (even borderline Quixotic) in our age, but many organisations try hard to keep complete control of all uses of their brand.
Worries can include:
- They might simply have unspecified concerns about a comms channel they know nothing about. They don't know if Behance is an "appropriate" place for their brand to be, they don't know what risks there might be - and they don't want to know and don't want to take the time to find out because it's not a priority to them. You could explain, but they won't know how to judge how far to trust your explanation. "If in doubt, say no" is a common attitude.
- If they were to notice a mistake later, or the content becomes out of date, they wouldn't want to feel the need to add your copy on Behance to the list of instances of the product that they would feel obliged to update (or count on you to update it in 3+ years when you might have moved on or been hit by a bus). They may be concerned that people could take your (now incorrect) copy, re-post it, and it'd look like they're putting out inaccurate content. (Of course, anyone can do this anyway, from any other copy... it's not always rational, sometimes people just "feel" like somehow allowing you to post it makes it their responsibility)
- They might want to (try to!) control comments on branded material someone affiliated with them posted. For example, they might worry that if someone posts a comment attacking them or promoting an untrue theory about them on a sort-of official posting by you and they don't respond, it might look like the attack has merit (believe me, people genuinely worry about things like this). They might also feel they would be obliged to "police" what you say about them under the item if it has their logo on it, since you could be seen as representing them.
- There may also be vague worries that you might link them to undesirable other work you do. For example, they might worry that if you make something inflammatory supporting a political cause, or do work for a very controversial client, and it appears on your profile next to them, they might be tarnished by association ("Local bakery hired designer of neo-Nazi logo"). Yes, it's incredibly unlikely - but some people do worry about things like this - and the less they understand sites like Behance, the more risk they'll imagine there is.
This all might be hard to understand if you grew up with social media and the noisey chaos of the internet. You might be thinking "But anyone can post anything, anywhere, at any time!".
But many people don't see it that way: many people are used to being able to control where their brand appears, expect to be able to control where their brand appears, and will instinctively say no to their brand appearing in a context they can't control.
You could argue they're like King Canute trying to hold back a rising tide, and in many cases I'd agree with you, but it's possible to understand why they feel the need to try.
As for reasoning with them: good luck, you'll have to somehow convince them that there's a business benefit to them that outweighs the risks they imagine. By far the best way is to have had a standard low-key line in your contract stating that you have the right to feature work in your online and offline portfolios (plural). Then it's their imagined risks, vs your real legal contract.