Are some ISPs using customer firewall?

Are some ISPs inserting a "customer" firewall for all, or maybe specific users when they connect up?

I ask because of checks with the Gibson Research site grc.com which you know gives a portscan check and shows you green/other colours if you have some type of firewall working/not working.

Trying different firewall rules on router I ran a check and showed green-full stealth, best result.

I disabled all rules and - still showed green.

No router.

I booted a Live CD with no running software firewall - green!

Another Live with software firewall and its own logging. Still grc.com gave a green and nothing notable in the log.

This was with plus.net and three 3G.

3 I believe don't provide a publicly routed ip address to you so you are effectively hidden by the network.
Could well be that some isp's block commonly attacked ports as it reduces unnecessary traffic on their network.

grc show that on open ports on mine too, and they're routed IPs with no NAT. Not surprisingly I don't trust their results.