Per Captain America’s skills’ description, he should be able to overwrite any tile with his maxed red/blue skills, but apparently not so?
I didn’t know this. I wonder if it treats Bubble tiles the same way? Thanks for the post. Never noticed this before. As to why…
If I understand it correctly, when a tile gets webbed in placed, it is essentially removed from play. Which is why other tiles fall around it when going down. If it’s not in play, it can’t be tageted.
Web tiles also are not considered special tiles. My “removed from play” theory is the reason for this also. 
edit.. this is just my theory, not something I know 100% for sure